Start stop containers 252

Fixes #252 
If there's a reason Gear units can't be manipulated concurrently, that should be handled in the Gear layer.  Later, when we have folks using Flocker to manage dozens or hundreds of containers we may want to limit the concurrency anyway (because starting 75 containers in parallel probably isn't a very nice thing to do to the host).

There are probably reasons *not* to serialize things at *this* layer though.  Starting an application isn't always going to just be "tell gear to start the unit".  Soon it's also going to be things like "wait for the volume to arrive".  We should be able to do much of this waiting concurrently.