(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.
This method is used in create_repos. It is abstracted into its own method to make testing easier, and also because it separates responsibilities more cleanly.

Create repos is used after the repos are signed to rerun createrepo in them.