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>
For sub-classes, there's no need to repeat documentation unless something's different.  Also, as a "rule-of-thumb" classes that have only  ``__init__`` and one other method (or less) normally is sign the class isn't needed.  Can the functionality  here be rolled into the ancestor?