subtests: Fix no need parent_subtests any more.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
Merge branch 'fix_parent_subtest' of https://github.com/jzupka/autotest-docker into jzupka-fix_parent_subtest

Conflicts:
	subtests/docker_cli/commit/commit.py

Fixed commit.py to use automatic prefix generating instead of config.
value.
Merge branch 'jzupka-fix_parent_subtest'