Fix glance create client unit tests
Fix glance create client unit tests

If tests test_create_glance_client_with_protocol_http and
test_create_glance_client_with_protocol_https from
cinder.tests.unit.image.test_glance.TestGlanceImageServiceClient are run
one after the other (regardless of the order) it will result in an
error: "AssertionError: Expected 'load_from_options' to be called once.
Called 0 times.".

The reason is that in cinder/image/glance.py we cache the session in
variable _SESSION, so the first test will set it and the second one will
reuse it and won't call the load_from_options method.

This patch solves this issue by resetting the caching variable in the
tests.

TrivialFix

Change-Id: Ia77d9d1a3aa4b54a08912584164739f851188784

Datera/cinder,openstack/cinder,Datera/cinder,phenoxim/cinder,j-griffith/cinder,j-griffith/cinder,phenoxim/cinder,openstack/cinder,mahak/cinder,mahak/cinder