Add known_stores option in class glance::api

In glance-api, if you use known_stores's default value and didn't use
all of the stores listed in known_stores, then glance-api service will
throw an error to point out which store missing, see more description
in related bug link.
This patchset adds known_stores option in glance::api, so that users
can configure it by their own needs.

Change-Id: If08254f419d182b82cc50137383985834f40a6fd
Closes-bug: #1259403
Merge "Add known_stores option in class glance::api"