Add PANTS_CONFIG_OVERRIDE to allow a second config file
Add PANTS_CONFIG_OVERRIDE to allow a second config file

* PANTS_CONFIG_OVERRIDE specifies a buildroot-relative config file path
* An env variable was chosen because pants.ini is loaded before argument
  parsing
* That file is loaded along with pants.ini
* If a value is in the override file it is chosen instead of pants.ini
* There's no cross-referencing between the files, so each file must be
  internally consistent

(sapling split of 5f2dddc496c406bd87c2f0d3d4ffb39452f4cb5c)
wisechengyi/pants,twitter/pants,landism/pants,kslundberg/pants,cevaris/pants,foursquare/pants,ericzundel/pants,fkorotkov/pants,pantsbuild/pants,areitz/pants,pombredanne/pants,manasapte/pants,tdyas/pants,peiyuwang/pants,benjyw/pants,tejal29/pants,peiyuwang/pants,15Dkatz/pants,jessrosenfield/pants,lahosken/pants,dgomez10/xanon,15Dkatz/pants,dbentley/pants,dgomez10/xanon,baroquebobcat/pants,Gabriel439/pants,foursquare/pants,wisechengyi/pants,lahosken/pants,mateor/pants,pombredanne/pants,15Dkatz/pants,UnrememberMe/pants,ity/pants,qma/pants,tdyas/pants,kslundberg/pants,TansyArron/pants,digwanderlust/pants,sameerparekh/pants,benjyw/pants,gmalmquist/pants,pombredanne/pants,square/pants,manasapte/pants,ericzundel/pants,slyphon/pants,tejal29/pants,scode/pants,ericzundel/pants,ericzundel/pants,twitter/pants,dgomez10/xanon,jsirois/pants,peiyuwang/pants,cevaris/pants,kslundberg/pants,kwlzn/pants,digwanderlust/pants,laurentgo/pants,megaserg/pants,tdyas/pants,pantsbuild/pants,kslundberg/pants,square/pants,UnrememberMe/pants,fkorotkov/pants,gmalmquist/pants,megaserg/pants,landism/pants,foursquare/pants,pgroudas/pants,kwlzn/pants,gmalmquist/pants,kwlzn/pants,foursquare/pants,UnrememberMe/pants,tejal29/pants,cevaris/pants,cevaris/pants,gmalmquist/pants,mateor/pants,jtrobec/pants,jessrosenfield/pants,benjyw/pants,gmalmquist/pants,landism/pants,scode/pants,pombredanne/pants,jessrosenfield/pants,Gabriel439/pants,kwlzn/pants,sid-kap/pants,lahosken/pants,slyphon/pants,sid-kap/pants,sid-kap/pants,baroquebobcat/pants,pombredanne/pants,ericzundel/pants,dgomez10/xanon,pombredanne/pants,slyphon/pants,manasapte/pants,pgroudas/pants,ity/pants,landism/pants,digwanderlust/pants,Gabriel439/pants,kslundberg/pants,tdyas/pants,TansyArron/pants,areitz/pants,kwlzn/pants,TansyArron/pants,dturner-tw/pants,gmalmquist/pants,15Dkatz/pants,Gabriel439/pants,slyphon/pants,landism/pants,twitter/pants,foursquare/pants,dturner-tw/pants,sid-kap/pants,pgroudas/pants,gmalmquist/pants,15Dkatz/pants,dturner-tw/pants,di0spyr0s/pants,fkorotkov/pants,twitter/pants,dbentley/pants,dbentley/pants,ity/pants,qma/pants,qma/pants,twitter/pants,landism/pants,wisechengyi/pants,wisechengyi/pants,jessrosenfield/pants,laurentgo/pants,tdyas/pants,cevaris/pants,UnrememberMe/pants,sameerparekh/pants,pgroudas/pants,laurentgo/pants,15Dkatz/pants,cevaris/pants,di0spyr0s/pants,laurentgo/pants,laurentgo/pants,fkorotkov/pants,benjyw/pants,sid-kap/pants,mateor/pants,peiyuwang/pants,kslundberg/pants,laurentgo/pants,dgomez10/xanon,slyphon/pants,mateor/pants,15Dkatz/pants,kwlzn/pants,ity/pants,lahosken/pants,TansyArron/pants,baroquebobcat/pants,qma/pants,di0spyr0s/pants,mateor/pants,areitz/pants,wisechengyi/pants,peiyuwang/pants,scode/pants,sameerparekh/pants,landism/pants,slyphon/pants,fkorotkov/pants,ericzundel/pants,peiyuwang/pants,dturner-tw/pants,fkorotkov/pants,twitter/pants,megaserg/pants,jsirois/pants,wisechengyi/pants,dbentley/pants,jtrobec/pants,sameerparekh/pants,pantsbuild/pants,di0spyr0s/pants,pgroudas/pants,manasapte/pants,twitter/pants,qma/pants,digwanderlust/pants,digwanderlust/pants,jessrosenfield/pants,kslundberg/pants,sameerparekh/pants,areitz/pants,baroquebobcat/pants,fkorotkov/pants,pantsbuild/pants,twitter/pants,dbentley/pants,jtrobec/pants,scode/pants,dgomez10/xanon,benjyw/pants,wisechengyi/pants,qma/pants,mateor/pants,megaserg/pants,dgomez10/xanon,square/pants,ericzundel/pants,manasapte/pants,dturner-tw/pants,tejal29/pants,TansyArron/pants,lahosken/pants,ity/pants,TansyArron/pants,dbentley/pants,sameerparekh/pants,fkorotkov/pants,UnrememberMe/pants,Gabriel439/pants,mateor/pants,peiyuwang/pants,jtrobec/pants,wisechengyi/pants,peiyuwang/pants,dgomez10/xanon,dturner-tw/pants,megaserg/pants,dgomez10/xanon,scode/pants,15Dkatz/pants,Gabriel439/pants,jessrosenfield/pants,foursquare/pants,digwanderlust/pants,tdyas/pants,TansyArron/pants,megaserg/pants,kwlzn/pants,foursquare/pants,tdyas/pants,baroquebobcat/pants,jsirois/pants,areitz/pants,pantsbuild/pants,baroquebobcat/pants,manasapte/pants,UnrememberMe/pants,jtrobec/pants,UnrememberMe/pants,cevaris/pants,qma/pants,foursquare/pants,tejal29/pants,baroquebobcat/pants,di0spyr0s/pants,megaserg/pants,sid-kap/pants,pgroudas/pants,lahosken/pants,square/pants,Gabriel439/pants,lahosken/pants,scode/pants,baroquebobcat/pants,dgomez10/xanon,landism/pants,dgomez10/xanon,sameerparekh/pants,scode/pants,UnrememberMe/pants,pantsbuild/pants,dbentley/pants,areitz/pants,benjyw/pants,foursquare/pants,pombredanne/pants,tdyas/pants,benjyw/pants,slyphon/pants,ity/pants,di0spyr0s/pants,mateor/pants,areitz/pants,lahosken/pants,jessrosenfield/pants,sid-kap/pants,ity/pants,jtrobec/pants,UnrememberMe/pants,jtrobec/pants,tdyas/pants,wisechengyi/pants,dturner-tw/pants,di0spyr0s/pants,twitter/pants,baroquebobcat/pants,pantsbuild/pants,ericzundel/pants,manasapte/pants