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 complained in ``icc`` test when attribute in ``conts`` was assigned.  Pylint or not,  my concern is mainly over keeping all tests uniform and consistent.  The convention of using ``stuff``/``sub_stuff`` allows run-time to have a totally separate name-space, guaranteed to never clash with future dockertest changes.  Additionally, when somebody else is looking for some piece of run-time data, there is no guessing needed where it is stored when everything is in one place.