Plugins may now register their own user preferences

To support our plugin system, I've added a stevedore hook that may be
used to load plugin values and their defaults from third parties.
This system initializes all plugins at startup, and decorates any
response with the discovered results on request. Loading is done via
stevedore, and I've started a new plugin package into which all of
our other plugins should probably be moved.

Change-Id: Idb7271dc37d01f36c512bbfd2be7f7b3c7b1a0c9
Merge "Plugins may now register their own user preferences"