Copy Ironic's database model codebase

The Ironic codebase is pretty simple for database access.  This work
leads into the introduction of versioned object technology from nova
and ironic that will be entering oslo:

https://review.openstack.org/#/c/127532/

This will drastically speed up the process of implementing moving the
RPC objects across the network.

Change-Id: I38aa451b658b66f5b6f10ced03ea2e0355af4ecd