Reimplementation of 6a3e1d98 commit

With the 6a3e1d98 commit NetworkManager was throwing an assertion so
we take a different approach when we create the bridge interface.
Also if the <interface-with-internet> is already a bridge interface
we skip creation.

The new approach is:

   1) Save the IPs and route table of INTERNET_IFACE
   2) If NetworkManager is running set INTERNET_IFACE as unmanaged
   3) Create BRIDGE_IFACE and add INTERNET_IFACE to it
   4) Set the previously saved IPs and route table to BRIDGE_IFACE

Better fix #19