Implement integration tests

[kolayne]

Add some tests to make sure docker-on-top works.

Ideally, tests should include:

• Basic use (creating a volume, using it in a container, deleting a volume) - ensure volumes function properly
• Simultaneous use of the same volume (mount volume into several containers, starting and terminating at different times, and ensure volumes function properly)
• Simultaneous use of two different volumes on top of the same base directory (run containers with different volumes on top of the same base directory, ensure their changes don't interfere, ensure changes from the host are propagated as they should)
• Use of volatile volumes (ensure volatile volumes function properly)
• Invalid user input (try to break stuff by providing invalid volume name, invalid base path, etc)
• Messing with the internal state (ensure docker-on-top won't allow invalid mounts/unmounts even when its internal state is messed with!)