pvecm addnode: ensure ring address isn't already used by cluster

pvecm addnode: ensure ring address isn't already used by cluster

If someone enters the wrong address by accident when adding a node it
may cause havoc in the cluster (meaning a reset of the whole cluster
when HA is used, may even happen more often during the recovery
tries. Also a whole lot of problems get triggered in gneral, even
witouth HA).

Further, user get into a hard to repair situation where a layman may
not be able to fix it by hand even when given directions by an
experienced user.

This is a really bad outcome for such a small and easy to make
mistake, so just make a small check and assert that the requested IPs
are not used by any node on any ring in the cluster configuration.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>