dockertest: Simplifies docker interface mechanism and adds config vars.

Simplifies docker interface mechanism from object composition to
module global variable assignation.
Adds variables in defaults.ini to set default test interface.

   default_container_iface = cli
   default_images_iface = cli
   default_dockercmd_iface = cli

example how to change default interface in whole test:
module global variable DockerImages

images.DockerImage = new interface class.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
dockertest: Adds dockercmd ability to implement other interfaces.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
dockertest: Move cmd status checker to output.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
subtests: Fix daemon tests to use interface without object container.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
subtests: Fix some tests.

Signed-off-by: Jiří Župka <jzupka@redhat.com>