Don't mock oslo.messaging _CallContext
Don't mock oslo.messaging _CallContext

This is a private method, rather mock the wrapping heat method.

Change-Id: I364723846b8851bbaec4f35aff3ccd708c40417f
Closes-bug: 1412836
(cherry-picked from d1ca5b6cccde982c95536498bfdcc75dbcecd1d9)

redhat-openstack/heat,redhat-openstack/heat