835977: Re-enable manifest signature checking.

The production/stage public candlepin certificate is now packaged in the
RPM. This is flagged as a config file but *without* noreplace, so any
deployment using their own upstream cert (we don't think there are any)
will have to restore it from the rpmsave file that results after a
candlepin rpm upgrade. This was done because we expect this to be the
desired behavior, there is no known usecase for using your own
upstream cert at this time.
Merge pull request #153 from candlepin/dgoodwin/sig-checking

835977: Re-enable manifest signature checking.