Allow specified backend capabilities to be retrieved

Allow specified backend capabilities to be retrieved

This change adds a new admin-api extension to allow admin to
fetch specified backend capabilities.
With this extension, admin can obtain what the current deployed
backend in Cinder is able to do from the endpoint.

The extension takes "host" as an parameter.
GET http://CINDER_API_ENDPOINT/v2/TENANT_ID/capabilities/<host>

DocImpact
Implements: blueprint get-volume-type-extra-specs
Change-Id: Ie8c3a3e554983f2e0819d52cf6d2db5efe7a0983