Goal: 

As a maintainer of networking-baremetal, following community decision to drop eventlet usage, the networking-baremetal HashRingMemberManager's notification listener must switch to an eventlet alternative.

Acceptance Criteria:

The function _set_up_listener should be updated to use a different, non-eventlet executor.

• Verify that the clustering of multiple networking-baremetal agent's operates as expected after changeing the executor.
  • Verify by running it in devstack, and launch multiple agent processes. It will build a cluster between the different processes, check the logs and verify that messages about agent members are joining the hash ring.
  • Above is a manual test, not something we have to automate.

NOTE: oslo.messaging library already supports an executor based on threading, as an alternative.