[WIP] Adjustments for SUSE manager for retail - Fine network setting

[Bischoff]

WORK IN PROGRESS - proof of concept, not finished

This PR adds support for private network between proxy ("branch server") and clients ("terminals"). It creates a second network interface on the proxy that connects to the clients. Both networks can communicate through routing, with masquerading enabled. A DHCP server on the proxy gives the addresses to the clients.

    +-----------+  10.160.0.0/16
    |           |
+---+----+   +--+----+
|  eth0  |   | eth0  |
|        |   |       |   +--------+   +--------+
| server |   | proxy |   | client |   | minion |
+--------+   |       |   |        |   |        |
             |  eth1 |   |  eth0  |   |  eth0  |
             +---+---+   +---+----+   +---+----+
                 |           |            |
                 +-----------+------------+  192.168.5.0/24

What is not finished yet:

1. help libvirt set the IP address of bridge on the real host to some address in the private network
2. as avahi does not span over networks, server and controller do not know about client.tf.local

Remarks:

1. addressed by this PR ; meanwhile, it can be worked around with

# ip address add 192.168.5.251/24 dev virbr1

2. addressed by (still) incomplete code in salt/default/unfinished

Apart from those details, it works. Major problem of this approach is that all DHCP / avahi / DNS setup that we do for sumaform to work has to be undone once we do retail for real.

Plan is to rebase this on top of #386.

[Bischoff]

Keeping this branch private only: closing.