nova imagebackend _base change
Closes-Bug: #1396334
Issue: nova _base images are stored outside of global shared space
       causes live migration to fail as the _base image wont be
       available.
Fix: _base is moved under global shared space so that migrating
     system will check the base image on the shared space and
     will initiate the migrate.
Tests: Verified the above fix in the testbed where issue is seen.

Change-Id: Icbd25e48153b2723ac686846b41b7d35f88a6a77