Use mock for HTTP calls in unit tests
Use mock for HTTP calls in unit tests

In test environment without network access, test_retry_policy_from_var
failed to run as it was not expecting the DNS lookup error. Mocking
the HTTP calls instead helps getting this pass.

Change-Id: I299ecbbc4f3aaafb2044ab32ab3a6bf9d1f9f9b1

openstack/mistral,openstack/mistral,StackStorm/mistral,StackStorm/mistral