distributors - UI

This pull-request adds the basis for being able to create "distributors" in katello. A distributor is a candlepin consumer type that can export a manifest.

This code is largely stand-alone from other areas and should not negatively impact existing features. I am adding this to master now so that it can be more widely tested and debug by candlepin group (from nightly builds).

Tests and additional refactoring to follow in subsequent pull-requests. Specifically, the tests will leverage the new minitest/vcr infrastructure.