host: Use docopt in flynn-host

Signed-off-by: Lewis Marshall <lewis@lmars.net>
controller: Set flynn-controller.app_name metadata

Signed-off-by: Lewis Marshall <lewis@lmars.net>
host: Add flynn-host ps command

Signed-off-by: Lewis Marshall <lewis@lmars.net>
host: Add flynn-host log command

Signed-off-by: Lewis Marshall <lewis@lmars.net>
host: Add flynn-host stop command

Signed-off-by: Lewis Marshall <lewis@lmars.net>