Configuration sanity checks for disabled regions

[johnctitus]

Taken from #91

Should we be concerend that Rackers may configure backups against an unsupported region? Perhaps there's some kind of validation we could do when they push a config, like calling the existing sanity check logic? That would at least point out that they've configured backups but no EC2 instances matched.

We should look into sanity checks to make sure we are not creating configurations for disabled regions.