Eliminate code duplication in discovery service when connecting to Rados

[gbregman]

In discovery.py we have the code:

        ceph_pool = self.config.get("ceph", "pool")
        ceph_conf = self.config.get("ceph", "config_file")
        rados_id = self.config.get_with_default("ceph", "id", "")
        conn = rados.Rados(conffile=ceph_conf, rados_id=rados_id)
        conn.connect()
        self.ioctx = conn.open_ioctx(ceph_pool)

This code is identical to what we have in state.py. We already ran into a bug which turned out to be about an update in state.py which wasn't done in discovery.py. we need to move this code to a separate function which will be called from both places.