Stub out unmocked keystone auth in signal tests

While these tests do setup a mock for keystoneclient, one of the tests
clears out these stubs before exitting to avoid needing to stub out
things stack_delete_after will do. However, stack_delete_after will
trigger a call to keystoneclient which leads to unmocked calls trying to
reach server.test from inside the tests. These failures go undetected
because they are after the test, and nothing is checking for error
events in this phase.

Change-Id: I608e0b5a263587d5151b4cb4ce3f03e289c4bdca
Partial-Bug: #1227962
Merge "Stub out unmocked keystone auth in signal tests"