Use testtools.matchers.GreaterThan in test_volumes_transfer

assertGreaterEqual is not in python 2.6 and while I know the long-term
direction for Tempest is to not support 2.6, this change is
super-trivial to make this work on 2.6.

Change-Id: Ie8315b539091f5fbff4e7dfd18a9b824f81a7fb7
Merge "Use testtools.matchers.GreaterThan in test_volumes_transfer"