Add new ec2 instance db API calls

The existing API calls for the EC2 instance 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 InstanceMapping nova object.

Change-Id: Ie1e5aa1c80c92cc65c6a1e062922f0375b742ea6
Merge "Add new ec2 instance db API calls"