Fix Invalid() exception msg
Fix Invalid() exception msg

resource.obj_name() is method, so in case of setting it as bound would
result in following message in the raised error [1]

[1] http://paste.openstack.org/show/793805/

Change-Id: Id5ffd3228824470ad79fe63fc6546b9ae5124776

mahak/cinder,mahak/cinder,openstack/cinder,openstack/cinder