(RAZOR-190) Add command to set repo source

Add an set-repo-source command that will allow the user to set the source of an existing repo. The user can also supply a 'refresh' option that can be true or false (defaults to true) that will force the repo to refresh its content from the new source.  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.