mpc_zone custom fact breaks on a docker server

[lynxnot]

The custom fact mpc_zone breaks when executed on a docker server.

Executing facter on a docker server gives the following result:

$ facter 
[...]
interfaces => docker0,eth0,eth1,eth2,lo,veth714185e
ipaddress => 172.17.42.1
ipaddress_docker0 => 172.17.42.1
ipaddress_eth0 => 10.102.68.xx
[...]

The custom fact is using the value of "ipaddress" to determine the zone, which in case docker is installed is wrong.

I'll make a PR with a possible solution for this bug.