dockertest.dockercmd: Add support to kill underlying process

This method links to the async_job's send_signal() function.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
subtests.docker_cli: Add detach test

This test executes bash and inside it the container. Than it tries
to detach and reattach it in various scenarios.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
subtests.docker_cli: Add interaction test

This test simulates user interaction with container running bash.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>