REP-77 - Validate service name in service list

**Note: Please don't pull this in at the moment.  -Seth**

So one of the things I don't like about this change is that the validation of the system model occurs in a different place from where the distributed datastore actually spins up. The distributed datastore service listens on the system model, and if it finds itself in the services list, it starts itself. Obviously, doing it that way necessitates that the validation occur separately (somewhere where we know about all of the services, and can check them all).

We can't do the check in the XSD because then Repose doesn't allow service extensibility. It would, however, be easiest to do it there using an enumeration of the services we provide (and then our configuration code would handle the rest -- 503s, fall-back, etc).

The best option for extensibility would be to do something like what we do for filters -- distribute packaged services and monitor them in some directory, then use a something (currently, a classloader is being used) to read the names of those services to compare against.

The solution provided here is a hacky way of performing the check, but ultimately, it's not functionally different from a XSD check. At best, we kill the new services enum later and move some of this code where it actually belongs.