Dynamic driver/server add/remove

[MattWindsor91]

Ideally, one should be able to add and remove drivers and servers from bra at run-time. This allows bra access to playout systems to be pulled down while the playout system is having maintenance run on it, for instance.

This could involve factoring out some of the Bra::Launcher code so it can be performed in-flight.

This will likely be a prerequisite to the task of making bra reload drivers that go down.

[MattWindsor91]

Here's a checklist of what we need (check only if in master):

• [ ] Allow configured modules to begin disabled (not started at load-time)
• [x] Make ModuleSet class, to store modules and take responsibility for stopping and starting them
• [ ] Move responsibility for owning modules to ModuleSet
• [ ] Make modules that are about to stop notify their ModuleSet so the ModuleSet can remove them
• [ ] Expose ModuleSets via model
• [ ] Add request handlers for dealing with PUT/POST/DELETEs on the model
• [ ] Stop BAPS driver from crashing BRA on failure
• [ ] (Optional) Add auto-reloading feature for ModuleSet.
• [ ] (Optional, debatable) Move creation/selection DSL to ModuleSets too
• [ ] (Optional, debatable) Allow configuration of modules at run-time too