(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 is not intuitive to me when referenced, e.g. on line 37, where I expect it to be a variable, not a method, and wonder why its not defined in scope.

I think it would make more sense to define this as a class instance variable, e.g.
```
module Pkg::Rpm::Repo
  class << self
    attr_reader :base_url
    @base_url = "http..."
    ...
   end
end
```