Implement better nova_admin_tenant_id_setter exists? method

Actually do some checking in the exists? method here to see if the
neutron.conf nova_admin_tenant_id setting is correct already.  That
way Puppet can properly recognize when the resource is already in
the right state.

Change-Id: I9692f4689683298cba4af1cacd700975f7e50b17
Closes-bug: 1392862
Merge "Implement better nova_admin_tenant_id_setter exists? method"