python docker version in u-c is bumped to 3.1.1 in [1].
Till 2.7.0 if MINIMUM_DOCKER_API_VERSION it just reported
warning but since 3.0.0 it reports Error, see [2]. This
patch bumps the expected version to 1.21 which is same
docker version used in magnum and supported by current
python docker.
[1] https://review.openstack.org/#/c/551428/
[2] df8422d079
Change-Id: Id93c5f70504c7d686dbd1b3d9bdfc1ef657d9287
yatin
dd7ed64690