transplant: Generate extra interfaces config file

Original work by Josep, only signed by Alex for upstreaming.

The DEA override may contain a IFCGF_<interface> section in its 'fuel:'
section, containing the necessary keys to produce a ifcfg-<interface>
file, like in this example:

fuel:
   IFCFG_ETH1:
     device: eth1
     ipaddress: 10.0.1.10
     netmask: 255.255.255.0
     gateway: 10.0.1.254

FIXME: In order for Network Manager to use the newly added interfaces
for outgoing traffic and honor their GATEWAY setting (e.g. if we just
added one public interface), the default route on admin iface (most of
the time called eth0) should be disabled. For now, we assume the admin
interface is always "eth0".

Change-Id: I96e3ac5046dd59ed10c43099dd76a6418806a1f3
Signed-off-by: Josep Puigdemont <josep.puigdemont@enea.com>
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>