Fixes #6097,bz1120427 - repository.feed -> repository.url

Migrating a repository data model attribute from feed to url. The word
feed applies to the feed url used by pulp, but in katello UI, CLI and
API we consistently use url for feed. To mitigate this confusion it was
deemed to rename feed to url.
Thats going to get tracked with issue -> http://projects.theforeman.org/issues/6671