Allow scheduler pool information to be retrieved

With pool support added to Cinder, now we are kind of in an awkward
situation where we require admin to input exact location for volumes
to-be managed (imported) or migrated, which must have pool info, but
there is no way to find out what pools are there for backends except
looking at the scheduler log.  That causes bad user experience, and
thus is a bug from UX POV.

This change simply adds a new admin-api extension to allow admin to
fetch all the pool information from scheduler cache (memory), which
closes the gap for end users.

This extension provides two level of pool information: names only or
detailed information:

Pool name only:
GET http://CINDER_API_ENDPOINT/v2/TENANT_ID/scheduler-stats/get_pools

Detailed Pool info:
GET http://CINDER_API_ENDPOINT/v2/TENANT_ID/scheduler-stats/get_pools
\?detail\=True

Closes-bug: #1364279

Change-Id: I445d4e472c83db2f2d8db414de139c87d09f8fda
Merge "Allow scheduler pool information to be retrieved"