Add new ec2 volume db API calls

The existing API calls for the EC2 volume ID mappings only returned
the EC2 ID or the UUID, not the full mapping.  We really want APIs
that return the whole mapping for use with a nova object.  This patch
adds those APIs which will be used by the VolumeMapping nova object.

Change-Id: I86361a557254dbddeae2ea49e986add813e5a934
Merge "Add new ec2 volume db API calls"