enhance support for virutal interfaces on remote machines

[Karl-Philipp Richter]

Currently `fwbuilder` doesn't seem to support virtual interfaces (e.g. one created by stanza `iface eth0:1 inet dhcp` and assigning it an address with `sudo ifconfig eth0:1 hw ether 00:22:22:ff:ff:ff && sudo dhclient eth0:1` (e.g. in `post-up`)) on a remote firewall. After the `fwbuilder` installation routine has been run remotely, the previously set up interface is gone and the installation script fails (example log attached).

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: fwbuilder 5.1.0-4
Uname: Linux 4.0.1-040001-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.17.2-0ubuntu1.1
Architecture: amd64
CurrentDesktop: Unity
Date: Sat Jun 6 14:08:05 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-04-02 (64 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Beta amd64 (20150326)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: fwbuilder
SystemImageInfo:
 current build number: 0
 device name:
 channel: daily
 last update: Unknown
UpgradeStatus: No upgrade log present (probably fresh install)