Create a networking service script, so we can run it after switchd

Create a networking service script, so we can run it after switchd

Ticket: CM-8790
Reviewed By: wkok,roopa
Testing Done: built, installed, rebooted

jessie's networking starts as an init.d service.   Trying to force ordering
between init.d and systemd services when there are dependencies doesn't work
well (especially since the init.d/networking service is forced very early
because of the remote filesystem requirement in jesie).

Converting networking to a script run as a systemd service allows us to start
networking after switchd.  The new script is /sbin/start-networking.  I chose
to keep it in /sbin, rather than put it in /usr/cumulus/bin, because it's core
functionaity.

I am not removing /etc/init.d/networking, it just gets ignored unless somebody
types it manually.  If somebody does that, systemctl runs through the lsb
hooks.  The two lost abilities below are just ignored if passed.  I'm
also preventing creating the rc.d symlinks to the init.d/networking
script to reduce future confusion.

We lose some init.d "convenience" functionality because it's not available
through systemd.  What we lose are:
  reload-currently-up - can still be done with ifreload --currently-up
  force-reload - can still be done with ifreload -f -a
We keep start, stop, reload, restart