enable ipv6 on an interface if we are configuring an address.

CentOS 7 has part of systemd disable ipv6 on all interfaces by default
if it isn't managing the interface.  This is probably not right, but
works for now.  systemd is going to be fun for everyone.
Don't pass nic because it is already there
Merge pull request #251 from galthaus/centos-07-ipv6-enable

enable ipv6 on an interface if we are configuring an address.