Optionally skip MAC address check in network

Network interfaces don't necessarily have a MAC address. For example,
the tunnel devices created by OpenVPN do not. Previously, passing a
network interface that did not have a MAC address caused the network
module to fail, since it assumed that there would be one.

This commit just adds a flag to the network module "mac", which defaults
to True. If True, the module behaves like before. If False, the check
for the MAC address is skipped and the {mac} format variable is replaced
with "NONE".

I tested this with my OpenVPN interface as well as my regular interface
and it works fine.