subtests.docker_cli: Fix rmi and tag indentation

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
subtests: Accept SubSubtest in *DockerCmd*

All SubSubtests should pass themselves to *DockerCmd* class now.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #259 from ldoktor/subsubtest

subtests: Accept SubSubtest in *DockerCmd*