[JENKINS-755] Describe the built-in JDK as '(System)'.
[JENKINS-755] Describe the built-in JDK as '(System)'.

As suggested in
https://issues.jenkins-ci.org/browse/JENKINS-755?focusedCommentId=206175&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-206175
this avoids confusion where users incorrectly think they're getting a
JDK they've listed in the configuration page.

patbos/jenkins,NehemiahMi/jenkins,evernat/jenkins,huybrechts/hudson,wuwen5/jenkins,tangkun75/jenkins,lilyJi/jenkins,aldaris/jenkins,tfennelly/jenkins,ydubreuil/jenkins,protazy/jenkins,jpbriend/jenkins,mdonohue/jenkins,azweb76/jenkins,jpbriend/jenkins,hplatou/jenkins,jglick/jenkins,ErikVerheul/jenkins,vjuranek/jenkins,amuniz/jenkins,samatdav/jenkins,vjuranek/jenkins,SebastienGllmt/jenkins,v1v/jenkins,FarmGeek4Life/jenkins,rsandell/jenkins,Jimilian/jenkins,ikedam/jenkins,godfath3r/jenkins,azweb76/jenkins,samatdav/jenkins,paulmillar/jenkins,jpederzolli/jenkins-1,batmat/jenkins,kzantow/jenkins,wuwen5/jenkins,lilyJi/jenkins,Ykus/jenkins,ChrisA89/jenkins,recena/jenkins,kzantow/jenkins,Ykus/jenkins,pjanouse/jenkins,ajshastri/jenkins,amuniz/jenkins,rsandell/jenkins,daniel-beck/jenkins,escoem/jenkins,csimons/jenkins,hplatou/jenkins,evernat/jenkins,MichaelPranovich/jenkins_sc,DanielWeber/jenkins,kohsuke/hudson,ydubreuil/jenkins,christ66/jenkins,Jochen-A-Fuerbacher/jenkins,tangkun75/jenkins,sathiya-mit/jenkins,kohsuke/hudson,stephenc/jenkins,ErikVerheul/jenkins,FarmGeek4Life/jenkins,andresrc/jenkins,dennisjlee/jenkins,hplatou/jenkins,kohsuke/hudson,lilyJi/jenkins,viqueen/jenkins,Jimilian/jenkins,bpzhang/jenkins,MichaelPranovich/jenkins_sc,ydubreuil/jenkins,batmat/jenkins,jpederzolli/jenkins-1,godfath3r/jenkins,ydubreuil/jenkins,sathiya-mit/jenkins,ndeloof/jenkins,duzifang/my-jenkins,bkmeneguello/jenkins,kohsuke/hudson,NehemiahMi/jenkins,huybrechts/hudson,dennisjlee/jenkins,recena/jenkins,Jimilian/jenkins,damianszczepanik/jenkins,viqueen/jenkins,bkmeneguello/jenkins,godfath3r/jenkins,ydubreuil/jenkins,amuniz/jenkins,amruthsoft9/Jenkis,msrb/jenkins,msrb/jenkins,duzifang/my-jenkins,jpbriend/jenkins,daniel-beck/jenkins,MarkEWaite/jenkins,fbelzunc/jenkins,mdonohue/jenkins,amruthsoft9/Jenkis,andresrc/jenkins,jglick/jenkins,Ykus/jenkins,amuniz/jenkins,daniel-beck/jenkins,FarmGeek4Life/jenkins,jpederzolli/jenkins-1,ErikVerheul/jenkins,kzantow/jenkins,jglick/jenkins,gitaccountforprashant/gittest,ikedam/jenkins,varmenise/jenkins,csimons/jenkins,batmat/jenkins,sathiya-mit/jenkins,damianszczepanik/jenkins,protazy/jenkins,ChrisA89/jenkins,kohsuke/hudson,duzifang/my-jenkins,vjuranek/jenkins,NehemiahMi/jenkins,dennisjlee/jenkins,ydubreuil/jenkins,stephenc/jenkins,mdonohue/jenkins,ikedam/jenkins,Vlatombe/jenkins,ndeloof/jenkins,ajshastri/jenkins,alvarolobato/jenkins,Ykus/jenkins,tangkun75/jenkins,SebastienGllmt/jenkins,jglick/jenkins,NehemiahMi/jenkins,aldaris/jenkins,kohsuke/hudson,fbelzunc/jenkins,duzifang/my-jenkins,pjanouse/jenkins,paulmillar/jenkins,Jimilian/jenkins,paulmillar/jenkins,duzifang/my-jenkins,stephenc/jenkins,damianszczepanik/jenkins,ikedam/jenkins,rsandell/jenkins,mdonohue/jenkins,dariver/jenkins,MarkEWaite/jenkins,rlugojr/jenkins,wuwen5/jenkins,protazy/jenkins,vjuranek/jenkins,amuniz/jenkins,amruthsoft9/Jenkis,ChrisA89/jenkins,Jochen-A-Fuerbacher/jenkins,batmat/jenkins,gitaccountforprashant/gittest,Vlatombe/jenkins,alvarolobato/jenkins,evernat/jenkins,FarmGeek4Life/jenkins,jpederzolli/jenkins-1,viqueen/jenkins,batmat/jenkins,recena/jenkins,ikedam/jenkins,MarkEWaite/jenkins,Vlatombe/jenkins,rsandell/jenkins,viqueen/jenkins,csimons/jenkins,aldaris/jenkins,andresrc/jenkins,MarkEWaite/jenkins,MichaelPranovich/jenkins_sc,rlugojr/jenkins,MarkEWaite/jenkins,Jochen-A-Fuerbacher/jenkins,daniel-beck/jenkins,azweb76/jenkins,ChrisA89/jenkins,dennisjlee/jenkins,ndeloof/jenkins,samatdav/jenkins,daniel-beck/jenkins,rlugojr/jenkins,oleg-nenashev/jenkins,samatdav/jenkins,gusreiber/jenkins,evernat/jenkins,christ66/jenkins,samatdav/jenkins,oleg-nenashev/jenkins,amruthsoft9/Jenkis,lilyJi/jenkins,ErikVerheul/jenkins,varmenise/jenkins,gitaccountforprashant/gittest,christ66/jenkins,v1v/jenkins,csimons/jenkins,christ66/jenkins,damianszczepanik/jenkins,samatdav/jenkins,msrb/jenkins,ndeloof/jenkins,pjanouse/jenkins,oleg-nenashev/jenkins,bkmeneguello/jenkins,pjanouse/jenkins,amruthsoft9/Jenkis,kzantow/jenkins,ydubreuil/jenkins,sathiya-mit/jenkins,andresrc/jenkins,viqueen/jenkins,azweb76/jenkins,SebastienGllmt/jenkins,aldaris/jenkins,protazy/jenkins,paulmillar/jenkins,msrb/jenkins,ajshastri/jenkins,FarmGeek4Life/jenkins,hplatou/jenkins,oleg-nenashev/jenkins,hplatou/jenkins,dariver/jenkins,tfennelly/jenkins,godfath3r/jenkins,ChrisA89/jenkins,kzantow/jenkins,Jochen-A-Fuerbacher/jenkins,wuwen5/jenkins,damianszczepanik/jenkins,Vlatombe/jenkins,gitaccountforprashant/gittest,jglick/jenkins,bkmeneguello/jenkins,bkmeneguello/jenkins,csimons/jenkins,viqueen/jenkins,gitaccountforprashant/gittest,godfath3r/jenkins,oleg-nenashev/jenkins,protazy/jenkins,alvarolobato/jenkins,fbelzunc/jenkins,olivergondza/jenkins,wuwen5/jenkins,daniel-beck/jenkins,MichaelPranovich/jenkins_sc,fbelzunc/jenkins,patbos/jenkins,amuniz/jenkins,tfennelly/jenkins,jenkinsci/jenkins,jenkinsci/jenkins,alvarolobato/jenkins,huybrechts/hudson,jpederzolli/jenkins-1,v1v/jenkins,kohsuke/hudson,paulmillar/jenkins,godfath3r/jenkins,wuwen5/jenkins,escoem/jenkins,amruthsoft9/Jenkis,duzifang/my-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