[lxc] add nic_opts to init function

Allows specification of mac, ipv4, and ipv6 for each nic for init function, e.g.:
lxc.init ... nic_opts='{"eth0": {"mac": "aa:bb:cc:dd:ee:ff", "ipv4": "10.1.1.1", "ipv6": "2001:db8::ff00:42:8329"}}'