Use ng-stop and ng-stats commands to kill buckd.
Use ng-stop and ng-stats commands to kill buckd.

Summary:
Earlier, we used `os.kill()` to kill the buckd process by pid, and wait for 10
seconds to check if a process with the same pid is still running before
terminating the process using `SIGKILL`.

Now, we use the `ng-stop` command to shutdown the nailgun server which blocks
until the server is shutdown.

Test Plan:
```
$ buckd
$ buckd --kill
```
Verify buckd is killed.

shybovycha/buck,clonetwin26/buck,dsyang/buck,janicduplessis/buck,neonichu/buck,grumpyjames/buck,daedric/buck,pwz3n0/buck,clonetwin26/buck,raviagarwal7/buck,rmaz/buck,stuhood/buck,vschs007/buck,sdwilsh/buck,Distrotech/buck,ilya-klyuchnikov/buck,janicduplessis/buck,brettwooldridge/buck,zpao/buck,justinmuller/buck,zhuxiaohao/buck,shybovycha/buck,k21/buck,ilya-klyuchnikov/buck,rowillia/buck,lukw00/buck,Distrotech/buck,pwz3n0/buck,raviagarwal7/buck,Addepar/buck,nguyentruongtho/buck,rowillia/buck,marcinkwiatkowski/buck,stuhood/buck,davido/buck,davido/buck,justinmuller/buck,mogers/buck,JoelMarcey/buck,liuyang-li/buck,mogers/buck,illicitonion/buck,MarkRunWu/buck,Addepar/buck,daedric/buck,JoelMarcey/buck,rowillia/buck,bocon13/buck,clonetwin26/buck,robbertvanginkel/buck,k21/buck,illicitonion/buck,clonetwin26/buck,brettwooldridge/buck,dpursehouse/buck,mnuessler/buck,vschs007/buck,SeleniumHQ/buck,artiya4u/buck,shybovycha/buck,sdwilsh/buck,davido/buck,SeleniumHQ/buck,clonetwin26/buck,shybovycha/buck,siddhartharay007/buck,robbertvanginkel/buck,rmaz/buck,dushmis/buck,clonetwin26/buck,k21/buck,shybovycha/buck,Addepar/buck,illicitonion/buck,darkforestzero/buck,stuhood/buck,illicitonion/buck,marcinkwiatkowski/buck,zhan-xiong/buck,mogers/buck,janicduplessis/buck,neonichu/buck,sdwilsh/buck,pwz3n0/buck,ilya-klyuchnikov/buck,bocon13/buck,hgl888/buck,Addepar/buck,shs96c/buck,raviagarwal7/buck,zpao/buck,vschs007/buck,dsyang/buck,tgummerer/buck,vine/buck,janicduplessis/buck,sdwilsh/buck,Distrotech/buck,shs96c/buck,dsyang/buck,rowillia/buck,grumpyjames/buck,tgummerer/buck,marcinkwiatkowski/buck,mnuessler/buck,shs96c/buck,vschs007/buck,liuyang-li/buck,OkBuilds/buck,grumpyjames/buck,romanoid/buck,raviagarwal7/buck,tgummerer/buck,ilya-klyuchnikov/buck,liuyang-li/buck,stuhood/buck,daedric/buck,marcinkwiatkowski/buck,vschs007/buck,1yvT0s/buck,luiseduardohdbackup/buck,luiseduardohdbackup/buck,rhencke/buck,artiya4u/buck,robbertvanginkel/buck,LegNeato/buck,janicduplessis/buck,bocon13/buck,grumpyjames/buck,tgummerer/buck,bocon13/buck,LegNeato/buck,luiseduardohdbackup/buck,Heart2009/buck,mikekap/buck,sdwilsh/buck,justinmuller/buck,lukw00/buck,davido/buck,k21/buck,justinmuller/buck,1yvT0s/buck,zhan-xiong/buck,rowillia/buck,OkBuilds/buck,robbertvanginkel/buck,tgummerer/buck,brettwooldridge/buck,Dominator008/buck,Addepar/buck,JoelMarcey/buck,Dominator008/buck,darkforestzero/buck,Heart2009/buck,shybovycha/buck,illicitonion/buck,rmaz/buck,robbertvanginkel/buck,mogers/buck,dpursehouse/buck,zhan-xiong/buck,ilya-klyuchnikov/buck,rhencke/buck,illicitonion/buck,artiya4u/buck,MarkRunWu/buck,brettwooldridge/buck,mikekap/buck,dushmis/buck,vine/buck,SeleniumHQ/buck,robbertvanginkel/buck,luiseduardohdbackup/buck,zhan-xiong/buck,robbertvanginkel/buck,dpursehouse/buck,nguyentruongtho/buck,lukw00/buck,janicduplessis/buck,illicitonion/buck,romanoid/buck,mread/buck,bocon13/buck,lukw00/buck,MarkRunWu/buck,justinmuller/buck,kageiit/buck,rmaz/buck,SeleniumHQ/buck,facebook/buck,vschs007/buck,siddhartharay007/buck,rowillia/buck,daedric/buck,shybovycha/buck,sdwilsh/buck,robbertvanginkel/buck,zpao/buck,pwz3n0/buck,k21/buck,raviagarwal7/buck,mogers/buck,k21/buck,stuhood/buck,rhencke/buck,JoelMarcey/buck,Dominator008/buck,vschs007/buck,grumpyjames/buck,dushmis/buck,ilya-klyuchnikov/buck,justinmuller/buck,zhan-xiong/buck,marcinkwiatkowski/buck,vine/buck,zhan-xiong/buck,clonetwin26/buck,1yvT0s/buck,sdwilsh/buck,rowillia/buck,MarkRunWu/buck,Learn-Android-app/buck,tgummerer/buck,nguyentruongtho/buck,zhuxiaohao/buck,romanoid/buck,justinmuller/buck,grumpyjames/buck,vine/buck,romanoid/buck,shs96c/buck,OkBuilds/buck,ilya-klyuchnikov/buck,stuhood/buck,Addepar/buck,LegNeato/buck,Learn-Android-app/buck,clonetwin26/buck,sdwilsh/buck,stuhood/buck,vine/buck,facebook/buck,vschs007/buck,lukw00/buck,mnuessler/buck,Dominator008/buck,shs96c/buck,siddhartharay007/buck,MarkRunWu/buck,janicduplessis/buck,JoelMarcey/buck,Distrotech/buck,Addepar/buck,rhencke/buck,raviagarwal7/buck,Heart2009/buck,raviagarwal7/buck,SeleniumHQ/buck,darkforestzero/buck,JoelMarcey/buck,dpursehouse/buck,ilya-klyuchnikov/buck,vine/buck,davido/buck,SeleniumHQ/buck,pwz3n0/buck,dpursehouse/buck,vine/buck,dushmis/buck,pwz3n0/buck,OkBuilds/buck,lukw00/buck,rhencke/buck,Distrotech/buck,daedric/buck,OkBuilds/buck,neonichu/buck,rmaz/buck,Dominator008/buck,zpao/buck,zhan-xiong/buck,Dominator008/buck,Learn-Android-app/buck,davido/buck,bocon13/buck,daedric/buck,bocon13/buck,dsyang/buck,daedric/buck,LegNeato/buck,mogers/buck,OkBuilds/buck,grumpyjames/buck,mnuessler/buck,daedric/buck,artiya4u/buck,grumpyjames/buck,zhan-xiong/buck,illicitonion/buck,luiseduardohdbackup/buck,rmaz/buck,darkforestzero/buck,justinmuller/buck,zhuxiaohao/buck,LegNeato/buck,clonetwin26/buck,liuyang-li/buck,JoelMarcey/buck,zhan-xiong/buck,SeleniumHQ/buck,shs96c/buck,brettwooldridge/buck,tgummerer/buck,SeleniumHQ/buck,liuyang-li/buck,1yvT0s/buck,pwz3n0/buck,illicitonion/buck,OkBuilds/buck,rhencke/buck,bocon13/buck,justinmuller/buck,nguyentruongtho/buck,siddhartharay007/buck,brettwooldridge/buck,mikekap/buck,LegNeato/buck,hgl888/buck,artiya4u/buck,SeleniumHQ/buck,mnuessler/buck,janicduplessis/buck,facebook/buck,MarkRunWu/buck,zpao/buck,zpao/buck,marcinkwiatkowski/buck,dsyang/buck,zhuxiaohao/buck,zhuxiaohao/buck,vine/buck,kageiit/buck,davido/buck,robbertvanginkel/buck,mikekap/buck,Dominator008/buck,davido/buck,rmaz/buck,dsyang/buck,vschs007/buck,illicitonion/buck,lukw00/buck,darkforestzero/buck,dsyang/buck,stuhood/buck,dsyang/buck,rhencke/buck,hgl888/buck,brettwooldridge/buck,Dominator008/buck,rmaz/buck,rmaz/buck,LegNeato/buck,Addepar/buck,SeleniumHQ/buck,artiya4u/buck,rmaz/buck,pwz3n0/buck,daedric/buck,JoelMarcey/buck,liuyang-li/buck,luiseduardohdbackup/buck,grumpyjames/buck,ilya-klyuchnikov/buck,sdwilsh/buck,stuhood/buck,shs96c/buck,kageiit/buck,Learn-Android-app/buck,shs96c/buck,artiya4u/buck,rmaz/buck,daedric/buck,Heart2009/buck,brettwooldridge/buck,davido/buck,neonichu/buck,mogers/buck,facebook/buck,clonetwin26/buck,vschs007/buck,hgl888/buck,facebook/buck,mikekap/buck,grumpyjames/buck,mogers/buck,dushmis/buck,marcinkwiatkowski/buck,Dominator008/buck,k21/buck,janicduplessis/buck,siddhartharay007/buck,daedric/buck,lukw00/buck,dsyang/buck,vschs007/buck,dsyang/buck,daedric/buck,romanoid/buck,k21/buck,dpursehouse/buck,zhan-xiong/buck,Dominator008/buck,shybovycha/buck,ilya-klyuchnikov/buck,clonetwin26/buck,siddhartharay007/buck,marcinkwiatkowski/buck,LegNeato/buck,Learn-Android-app/buck,mogers/buck,brettwooldridge/buck,romanoid/buck,rowillia/buck,Learn-Android-app/buck,Distrotech/buck,darkforestzero/buck,Learn-Android-app/buck,mikekap/buck,janicduplessis/buck,vine/buck,dsyang/buck,zhan-xiong/buck,shs96c/buck,OkBuilds/buck,romanoid/buck,hgl888/buck,Learn-Android-app/buck,brettwooldridge/buck,mikekap/buck,luiseduardohdbackup/buck,Distrotech/buck,luiseduardohdbackup/buck,grumpyjames/buck,Dominator008/buck,darkforestzero/buck,bocon13/buck,tgummerer/buck,lukw00/buck,daedric/buck,JoelMarcey/buck,shs96c/buck,k21/buck,justinmuller/buck,Heart2009/buck,mread/buck,raviagarwal7/buck,romanoid/buck,LegNeato/buck,OkBuilds/buck,LegNeato/buck,dushmis/buck,darkforestzero/buck,ilya-klyuchnikov/buck,shybovycha/buck,neonichu/buck,SeleniumHQ/buck,rhencke/buck,raviagarwal7/buck,mnuessler/buck,OkBuilds/buck,tgummerer/buck,MarkRunWu/buck,kageiit/buck,mikekap/buck,Distrotech/buck,mikekap/buck,justinmuller/buck,neonichu/buck,dpursehouse/buck,mogers/buck,JoelMarcey/buck,neonichu/buck,k21/buck,Learn-Android-app/buck,JoelMarcey/buck,liuyang-li/buck,romanoid/buck,liuyang-li/buck,vschs007/buck,Learn-Android-app/buck,bocon13/buck,tgummerer/buck,vine/buck,Heart2009/buck,romanoid/buck,facebook/buck,Addepar/buck,liuyang-li/buck,illicitonion/buck,bocon13/buck,facebook/buck,marcinkwiatkowski/buck,shs96c/buck,illicitonion/buck,rhencke/buck,stuhood/buck,kageiit/buck,shybovycha/buck,artiya4u/buck,dpursehouse/buck,davido/buck,Heart2009/buck,zhuxiaohao/buck,mnuessler/buck,JoelMarcey/buck,neonichu/buck,dushmis/buck,mnuessler/buck,mikekap/buck,artiya4u/buck,mikekap/buck,janicduplessis/buck,romanoid/buck,zhan-xiong/buck,darkforestzero/buck,justinmuller/buck,OkBuilds/buck,shybovycha/buck,rhencke/buck,zhuxiaohao/buck,hgl888/buck,artiya4u/buck,sdwilsh/buck,Heart2009/buck,tgummerer/buck,davido/buck,SeleniumHQ/buck,nguyentruongtho/buck,mogers/buck,hgl888/buck,grumpyjames/buck,hgl888/buck,shs96c/buck,mnuessler/buck,mread/buck,raviagarwal7/buck,siddhartharay007/buck,marcinkwiatkowski/buck,sdwilsh/buck,brettwooldridge/buck,OkBuilds/buck,1yvT0s/buck,Heart2009/buck,pwz3n0/buck,darkforestzero/buck,luiseduardohdbackup/buck,shs96c/buck,siddhartharay007/buck,zpao/buck,rowillia/buck,lukw00/buck,kageiit/buck,1yvT0s/buck,robbertvanginkel/buck,davido/buck,ilya-klyuchnikov/buck,marcinkwiatkowski/buck,LegNeato/buck,Dominator008/buck,romanoid/buck,LegNeato/buck,justinmuller/buck,pwz3n0/buck,dushmis/buck,liuyang-li/buck,kageiit/buck,sdwilsh/buck,raviagarwal7/buck,rhencke/buck,stuhood/buck,dsyang/buck,bocon13/buck,clonetwin26/buck,Distrotech/buck,marcinkwiatkowski/buck,sdwilsh/buck,dsyang/buck,raviagarwal7/buck,dushmis/buck,rowillia/buck,illicitonion/buck,rmaz/buck,MarkRunWu/buck,Learn-Android-app/buck,brettwooldridge/buck,nguyentruongtho/buck,k21/buck,davido/buck,janicduplessis/buck,darkforestzero/buck,darkforestzero/buck,zhan-xiong/buck,1yvT0s/buck,mikekap/buck,pwz3n0/buck,dushmis/buck,liuyang-li/buck,robbertvanginkel/buck,k21/buck,SeleniumHQ/buck,Addepar/buck,clonetwin26/buck,siddhartharay007/buck,LegNeato/buck,vine/buck,raviagarwal7/buck,vschs007/buck,rowillia/buck,romanoid/buck,1yvT0s/buck,Distrotech/buck,Addepar/buck,mread/buck,mread/buck,darkforestzero/buck,rmaz/buck,zhuxiaohao/buck,shybovycha/buck,1yvT0s/buck,Addepar/buck,brettwooldridge/buck,robbertvanginkel/buck,OkBuilds/buck,mread/buck,zhuxiaohao/buck,robbertvanginkel/buck,nguyentruongtho/buck,hgl888/buck,artiya4u/buck,marcinkwiatkowski/buck,Addepar/buck,shybovycha/buck,rowillia/buck,JoelMarcey/buck,k21/buck,lukw00/buck,neonichu/buck,Distrotech/buck,ilya-klyuchnikov/buck