Enable py3 mock.patch of RuntimeError
Enable py3 mock.patch of RuntimeError

RuntimeError is moved to builtins in Python3.x so adjust
the @mock.patch accordingly when running in Python3.x.

Change-Id: I5e1ce3ed40add635ce25bc1d80f88dc7ad9d2c59
Closes-Bug: #1522551

openstack/oslo.service