fixes #6204 / BZ 1099016 - Fix gpg key on repo create and yum retrieval

This commit addresses 2 issues:
1. update the repo create to support associating the gpg key url
   in candlepin
2. update the v2 api to expose the 'gpg_key_content' api to enable
   yum clients to retrieve the key
Merge pull request #4263 from bbuckingham/issue-6204

fixes #6204 / BZ 1099016 - Fix gpg key on repo create and yum retrieval