Remove redundant _create_instance method from test_compute

The _create_instance method in test_compute.BaseTestCase simply calls
through to _create_fake_instance, so it's redundant. Furthermore, the
docstring was wrong, it said it returned a UUID when it really returned
the instance reference. This patch simply removes _create_instance and
replaces it's usage with _create_fake_instance.

Fixes bug 1206993

Change-Id: I3fc42c94691cd0d046f19cdf2110a95b9a125edd
Merge "Remove redundant _create_instance method from test_compute"