Optional / fallback usage of internal cloud IP addresses if available

[jmrk84]

There are some situations in which the external IPs cannot be used for SSH (eg due to firewall rules), making it desirable to have this as a configuration option.

Reference to original discussion:

is there any particular reason why the external IP is currently being used?

Not all clouds support multiple IPs or do allocate multiple IP addresses automatically.

I guess we can try to expose ElastiCluster's idea of "preferred/connection IP" as the "public IP" and then assume other IP addresses (if any) should be used for intra-cluster communication... (But what should ElastiCluster do if there are more than 1 "internal" IP addresses?)

Anyway, I think this effort belongs in a different issue :-) merging pause/restart should not wait until the public/private IP support is implemented...

Originally posted by @riccardomurri in https://github.com/gc3-uzh-ch/elasticluster/issues/583#issuecomment-466925268