(CPR-52) Add support for the automatic building of nightly signed repos

This PR defines a small stack of rake tasks that handle the signing and shipping of repos for a nightly repo. In the process several rake tasks were moved into the appropriate libraries and some additional utilities were added. This PR also deprecates the gpg_name config setting in favor of the gpg_key, and adds a new signing_server config setting for use in automatic signing.
While we're in here, can we change this to `modern_rpms = all_rpms - old_rpms`? Or at least do something else here to make this whole thing need less maintenance when we add/remove platforms?