Fix: ceph: mon_address not considered by new MON

Fix: ceph: mon_address not considered by new MON

The public_addr option for creating a new MON is only valid for manual
startup (since Ceph Jewel) and is just ignored by ceph-mon during setup.
As the MON is started after the creation through systemd without an IP
specified. It is trying to auto-select an IP.

Before this patch the public_addr was only explicitly written to the
ceph.conf if no public_network was set. The mon_address is only needed
in the config on the first start of the MON.

The ceph-mon itself tries to select an IP on the following conditions.
- no public_network or public_addr is in the ceph.conf
    * startup fails

- public_network is in the ceph.conf
    * with a single network, take the first available IP
    * on multiple networks, walk through the list orderly and start on
      the first network where an IP is found

Signed-off-by: Alwin Antreich <a.antreich@proxmox.com>