Add verification and fallback to cluster join/addnode

Add verification and fallback to cluster join/addnode

Verify that the config of the new node is valid and compatible with the
cluster (i.e. that the links for the new node match the currently
configured nodes).

Additionally, fallback is provided via a new parameter to addnode,
'new_node_ip'. Previously, fallback was handled on the joining node, by
setting it's local IP as 'link0', however, a cluster with only one link,
but numbered 1-7 is still valid, and a fallback is possible, but the old
code would now fail.

Instead, pass the locally resolved IP via a seperate parameter
(resolving the IP on the cluster side is impractical, as IP resolution
could fail or provide a wrong IP for Corosync).

For compatibility reasons, allow fallback to occur via the old
method as well, but mark with FIXME for future removal.

Fallback fails in case the cluster has more than one link, in this case
only the user can know which NIC/IP corresponds to which cluster link.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>