Add local selenium config file. Fixes #3118.

  Selecting the --local option, or setting PERSONA_NO_SAUCE=1 in the
  environment, will now switch to a different config file. The set of
  supported platforms will be totally generic.
Show how to run >1 firefoxen locally. Fixes #3138
Improve explanation of using multiple Firefoxen
Merge pull request #3142 from 6a68/issue_3118_local_selenium

Add separate selenium config for local runs, including multiple FF channels. Fixes #3118, #3138.

AHHA! I get it now. I totally dig this.

r+