ipv4 mtu field and ipv6 interface fields and post_install_network_config snippet ipv6 interface configuration

[Jon Sabo]

I would like to be able to configure ipv6 network settings and set the ipv4
mtu setting in cobbler.  Attached is a patch that I hope could be included
in some future release of cobbler.  Please take a look when you get a
chance.  I am not good with git at all so if this does not apply I'll try
and figure out why and send an updated patch.

Thanks,

Jonathan