dockertest.images & containers: Only check SubBase

Similar to change made for DockerCmd, to allow instances
implementing the common aspects of Subtest and SubSubtest
to be used interchangably.

Signed-off-by: Chris Evich <cevich@redhat.com>
Merge branch 'img_cnt_subbase' of https://github.com/cevich/autotest-docker

Conflicts:
	dockertest/images.py