Update ferm rules to support IPv6

New firewall configuration should work with IPv6 network.

'postfix_allow_*' variables have been changed slightly: by default they
are empty lists and allow for connections from any host/network. If
hosts or networks are specified, then firewall will limit connections
only to these hosts/networks.
Remove tags from role dependencies
Restart ferm "manually" when firewall rules change

I'm not sure why, but ferm handler does not want to "fire" when notified
by Postfix on firewall rules change. Perhaps the reason is both ferm and
postfix roles being in the common playbook together? No idea. This
should fix the issue for now.
Merge pull request #3 from drybjed/postfix-updates

Postfix updates