Add ability to launch headless containers and attach to running containers to CLI

[ChrisTimperley]

The following commands should be added to the bugzoo container command group:

• $ bugzoo container ls: should produce a list of running containers.
• $ bugzoo container info [container]: should provide information about a running container.
• $ bugzoo container launch -d [snapshot]: should spawn a headless container that persists beyond the lifetime of the CLI process.
• $ bugzoo container kill [snapshots...]: should destroy a running container.
• $ bugzoo container exec [container] [command]: should execute a given container inside a specified container.