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.

Change-Id: I943cbb017ccdbe4f6c994bc83cbefe8693f4672b
Fixes: bug #1185178.