Adds PUBLICLY_FORWARDED_PORTS for SubnetTier

- A config key that can be set on an entity within a SubnetTier,
  where the config lists a set of attributes indicating ports to be
  opened. When these attributes are set, it will openPortForwarding
  for the given port, and advertise the public endpoint as a new
  attribute on the entity.
  Thus one can describe a blueprint in pure YAML
port-forwarded machines: don’t wait for ssh-reachable if WAIT_FOR_SSHABLE=false
SubnetTier support for vcloud-director