Improve method to get list of active conductors

To facilitate the consistent hash ring, we must know not only what
conductors are active, but also which driver(s) each active conductor
supports. This patch renames list_active_conductor_drivers to
get_active_driver_dict and changes the return value to be a dict,
mapping drivers to the set of hosts which support them.

blueprint instance-mapping-by-consistent-hash

Change-Id: Ie327aa7d9207f6f5da1380631dbbc431dfd7e218
Merge "Improve method to get list of active conductors"