dockertest: Speed up subsubtest config creating by deep copy-able version.

Creation of subsubtests instance took quite a long time because
deep copy of configuration took a long time.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
Merge pull request #394 from jzupka/config_speedup

dockertest: Speed up subsubtest config creating by deep copy-able version.