Long running process 884

Fixes #884 

This lays the groundwork for a long-running volume manager process.  Basically, it gives us `flocker-serve` script and a place to stick a service (such as a TLS server exposing an HTTP API for controlling the volume manager) to be run by that command.

It was pretty nice not to have to think about the specific way in which a `VolumeService` must be created.  So hooray for our abstractions, I say. :smile: