subtests.docker_daemon: Adds tests for connection with certificate.

Tests generates certificates and sign them up with created CA certificate,
then test connect server and client part with different configurations.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
It's helpful to give a prefix/suffix that points at where/why it was made.  Then when/if there are leftovers that don't get cleaned, it's easy to see where they came from.  Hmmm, that might be a good update to make separately into ``get_unique_name`` directly (since it has access to the ``subtest`` already).