Add to_dict() method to apiclient Resource

Resource class in different openstack clients (ceilometerclient,
glanceclient, heatclient, ironiccilent, tuskarclient) has to_dict()
method so it would be nice to have this method in common code to
avoid code duplication.

blueprint common-client-library-2

Change-Id: I1db6c12a1f798de7f7fafd0c34fb0ef523610153
Merge "Add to_dict() method to apiclient Resource"