IMP-587: Webserver should not default to /tmp/ for document root
IMP-587: Webserver should not default to /tmp/ for document root

A world-writeable document root is a bad idea, because a malicious user can
write symlinks to that folder that point to files readable only by impalad's
user. The user can then read these files through the web server.

This patch makes the default doc root empty if IMPALA_HOME is not set; and an
empty doc root means don't serve files. There's also an explicit flag to disable
file serving.

gerashegalov/Impala,ibmsoe/ImpalaPPC,cloudera/recordservice,bowlofstew/Impala,gistic/PublicSpatialImpala,AtScaleInc/Impala,bowlofstew/Impala,theyaa/Impala,scalingdata/Impala,grundprinzip/Impala,XiaominZhang/Impala,tempbottle/Impala,cgvarela/Impala,bratatidas9/Impala-1,rdblue/Impala,mapr/impala,cgvarela/Impala,mapr/impala,cchanning/Impala,tempbottle/Impala,henryr/Impala,bowlofstew/Impala,rdblue/Impala,XiaominZhang/Impala,henryr/Impala,cloudera/recordservice,kapilrastogi/Impala,XiaominZhang/Impala,AtScaleInc/Impala,caseyching/Impala,caseyching/Impala,cchanning/Impala,gistic/PublicSpatialImpala,andybab/Impala,placrosse/ImpalaToGo,kapilrastogi/Impala,XiaominZhang/Impala,lirui-intel/Impala,gerashegalov/Impala,placrosse/ImpalaToGo,ibmsoe/ImpalaPPC,cloudera/recordservice,placrosse/ImpalaToGo,cchanning/Impala,AtScaleInc/Impala,caseyching/Impala,caseyching/Impala,lirui-intel/Impala,ImpalaToGo/ImpalaToGo,bowlofstew/Impala,theyaa/Impala,theyaa/Impala,grundprinzip/Impala,AtScaleInc/Impala,rampage644/impala-cut,rdblue/Impala,kapilrastogi/Impala,gerashegalov/Impala,gerashegalov/Impala,cloudera/recordservice,ImpalaToGo/ImpalaToGo,bowlofstew/Impala,cchanning/Impala,scalingdata/Impala,ibmsoe/ImpalaPPC,tempbottle/Impala,cgvarela/Impala,lirui-intel/Impala,ImpalaToGo/ImpalaToGo,mapr/impala,brightchen/Impala,lnliuxing/Impala,XiaominZhang/Impala,grundprinzip/Impala,brightchen/Impala,rampage644/impala-cut,AtScaleInc/Impala,henryr/Impala,ibmsoe/ImpalaPPC,bratatidas9/Impala-1,cloudera/recordservice,andybab/Impala,bratatidas9/Impala-1,lnliuxing/Impala,rampage644/impala-cut,ImpalaToGo/ImpalaToGo,andybab/Impala,tempbottle/Impala,cloudera/recordservice,tempbottle/Impala,bratatidas9/Impala-1,brightchen/Impala,theyaa/Impala,lnliuxing/Impala,cchanning/Impala,cloudera/recordservice,ibmsoe/ImpalaPPC,gistic/PublicSpatialImpala,scalingdata/Impala,theyaa/Impala,gistic/PublicSpatialImpala,rdblue/Impala,bratatidas9/Impala-1,bratatidas9/Impala-1,andybab/Impala,cgvarela/Impala,theyaa/Impala,grundprinzip/Impala,ImpalaToGo/ImpalaToGo,rampage644/impala-cut,rampage644/impala-cut,caseyching/Impala,placrosse/ImpalaToGo,scalingdata/Impala,lirui-intel/Impala,ibmsoe/ImpalaPPC,brightchen/Impala,lirui-intel/Impala,bowlofstew/Impala,bowlofstew/Impala,placrosse/ImpalaToGo,cgvarela/Impala,scalingdata/Impala,AtScaleInc/Impala,gistic/PublicSpatialImpala,mapr/impala,placrosse/ImpalaToGo,brightchen/Impala,XiaominZhang/Impala,lnliuxing/Impala,cgvarela/Impala,grundprinzip/Impala,gerashegalov/Impala,henryr/Impala,theyaa/Impala,tempbottle/Impala,kapilrastogi/Impala,scalingdata/Impala,rampage644/impala-cut,andybab/Impala,lnliuxing/Impala,brightchen/Impala,lnliuxing/Impala,caseyching/Impala,brightchen/Impala,grundprinzip/Impala,XiaominZhang/Impala,gerashegalov/Impala,bratatidas9/Impala-1,lirui-intel/Impala,mapr/impala,gistic/PublicSpatialImpala,andybab/Impala,cchanning/Impala,lnliuxing/Impala,ibmsoe/ImpalaPPC,kapilrastogi/Impala,rdblue/Impala,gerashegalov/Impala,kapilrastogi/Impala,cgvarela/Impala,rdblue/Impala,henryr/Impala,caseyching/Impala,lirui-intel/Impala,tempbottle/Impala,cchanning/Impala,henryr/Impala,kapilrastogi/Impala,ImpalaToGo/ImpalaToGo,rdblue/Impala