Rename glance.store to glance_store

In an effort to avoid adding more namespace packages[1], the project
formerly known as glance.store is now glance_store[2].

[1] http://lists.openstack.org/pipermail/openstack-dev/2014-August/044194.html
[2] http://lists.openstack.org/pipermail/openstack-dev/2014-August/044355.html

Change-Id: I5b741191038a958bb8df0d6b72553e37f6fe0a6a
Merge "Rename glance.store to glance_store"