main.py: Fixed argument parsing error.
tests: New approach when setting the test environment. It now creates a
virtual network interface, with two devices.

As a future work, this must be configurable from a config file.

.travis.yml: it has been deleted for it seems not possible to get
`vde_swtich` running on its virtual machine.
Syntax checking is now performed on all python files.

The pylint errors that need to be disabled in a whole file, must be
indicated in `check_code.sh`.

Some syntax errors needed fixing to get the `make check-pylint` passing.