pvecm add: assert that ringX IPs are available on node add

pvecm add: assert that ringX IPs are available on node add

If 'ringX_addr' parameters are used on adding a node to a cluster
check if those addresses are actually configured on the to-be-added
node. It makes no sense that the address is not or multiple times
configured.

This prevents a node in limbo, waiting for quorum (if it was the
second node in a cluster, even two node would be in the no-quorum
limbo) where manual pmxcfs kills, local starts and manual
configuration edits which may need to get manually synced to other
cluster members are needed.

The check does not cost much and gets only made on node additions, so
assert with our get_local_ip_from_cidr method that the IP is
configured on any interface.

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