Disable cluster init with a reachable IP

Make the cluster init fail if the IP to which the cluster name resolved
is already reachable by the master node. This is not a foolproof
solution, but it allows a cheap method of detecting simple mistakes.

It will also disallow using the master node name as cluster name (which
is something good).

The only drawbacks that I see are:
  - you are not allowed to do this, which might come in handy in cluster
    upgrades; but since we support rename, this is mitigated
  - cluster init takes longer now (+the timeout value, set to 5
    seconds), but since this is a one-off operation, it should be ok

Reviewed-by: ultrotter