ensure priorities for custom repos as well

Unfortunately, custom repos where getting overridden even if they had `piority=1` because ceph-deploy was not passing that option and was not installing the plugin.

This enforces that behavior but only when the priority flag is set.

Reference ticket: http://tracker.ceph.com/issues/9645 
these where changed from `get` to `pop` so that we could re-use `**kw` and not have duplicate keywords.