ceph-nvmeof-mon fixes

ceph-nvmeof-mon fixes

Resolves: rhbz#2279862

- ceph-nvmeof-mon: nvme-gw create/delete
  * move creation to the `daemon_check_post` method to prevent zombie creation.
    this change aligns with the dashboard logic, which uses the same callback.
  * implement `purge` method to ensure that no zombie gateways are left behind
    once the service is removed.
  Signed-off-by: Alexander Indenbaum <aindenba@redhat.com>
- nvmeof service spec: enable_monitor_client by default
  Signed-off-by: Alexander Indenbaum <aindenba@redhat.com>
- fix gw stuck in  ana-state GW_WAIT_FAILBACK_PREPARED
  Signed-off-by: Leonid Chernin <leonidc@il.ibm.com>
Signed-off-by: Alexander Indenbaum <aindenba@redhat.com>
(cherry picked from commit b0c764b6e22fbb51f59e469d9dd99895c152f73e)