Remove sysctl configuration from debops.lxc

Settings in sysctl are more related to the firewall and packet
forwarding rather than to LXC, and will be configured elsewhere.
Add stricter forwarding rules for containers

Packet forwarding in firewall will be now more strict, and it can also
be disabled in case you want to setup your own firewall rules.
Merge pull request #3 from drybjed/lxc-update

Update firewall support