Autodetect the default network interface

[linkvt]

Closes #13

Hi,

this PR auto detects the default network interface as suggested in #13.

I thought that this could be done with the PYTHON2BASH configuration option but it is interpreted with the python function ast.literal_eval in a script in the base image which only allows safe python data types, so this was not possible and the implementation here was required.

I manipulated the check to make it fail and the container didn't start so everything seems to be working.

Thanks, Vincent