[PR] Clean up GPG signing key handling

Use the work from @mirmillo as a base to clean up gpg key stuff, see #446.

* We're able to use `hkp` for port 80 requests to work around issues with restrictive proxies.
* We were actually using the wrong signing key for Nginx. This we can grab directly from nginx.org and apply as we go.

Total bonus - no more gpg red. :)