Validate inter-node connectivity as a prerequisite

[NicolasT]

Early on in the deploy (and related) playbooks, where we check all nodes are reachable from the deployment host (ping), we should figure out a way to validate all nodes can talk to each other as well, e.g. to make sure no overly restrictive OpenStack security group is in place.

[NicolasT]

This is partially handled by #368, but doing TCP-level checks would be useful as well.