Adding the Sauce OnDemand Plugin

This change adds support for the sauce ondemand plugin.  Using
this plugin you can actually set everything at the job config
level and not touch the global config.  I did have to put a note
in about how to know what to put for the platform since the
list of platforms is dynamically created by the plugin by
hitting the sauce labs site to get the current list, so I wasn't
able to create a dictionary of what you see in the config and
what the valid values are for the yaml.

Change-Id: I3579111af0b18967cd09f28a5388e6637fa766d5