Add separate selenium config for local runs, including multiple FF channels. Fixes #3118, #3138.

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.

Example:

````bash
]$ ./scripts/run-all.js --local --list-platforms
5 platforms configured:
  * firefox
  * chrome
  * opera
  * safari
  * ie
]$ ./scripts/run-all.js --list-platforms
9 platforms configured:
  * win7_firefox_16
  * linux_firefox_16
  * osx_firefox_14
  * win7_chrome
  * winxp_ie_8
  * win7_ie_9
  * win8_ie_10
  * linux_opera_12
  * osx_safari_5
````

Updated to include an example of how to specify an additional firefox platform locally. Specifying multiple channels for chrome is a bit tougher--I couldn't figure out the syntax in 30 min of twiddling--but we really only need multiple firefoxen to close out the bug :-)