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