Fix Storage Backend Wording

The previous wording was confusing, the config setting does not
configure the *only* glance backend but the *default* glance
backend. You can still overwrite it per image, and searching
for glance images involves all configured backends.
Merge pull request #132 from dirkmueller/default_store

+1
RadosGW support for Cinder - intergration with internal Ceph