(RAZOR-190) Add command to update repo

Add an update-repo command that will allow the update of repo settings. When the supplied setting are different, the process will trigger a 'make_repo_available' event.  This is required, because if you create the repo and then policies/tags etc only to realise you botched the definition of the repo, you first have to pull down all the dependants (policies and the like) to delete/recreate the repo.