test_glance.py: Stub out _get_member_model as well.

glanceclient's v2 client needs to also have its _get_member_model()
stubbed out as well, to avoid attempts to reach 'fake_host' during
client instantiation.

Update: Only stub when _get_member_model() when required.

Fixes: bug #1185178.

Cherry picked from https://review.openstack.org/#/c/30795

Change-Id: Ida47b3039c2539103fa3394d1443c896d7a81135