nose is no longer maintained, thus we have to move to another
testing system. This commit updates the tox setup and all tests
to use pytest instead of nose.
The --compat call will trigger the call of the kiwicompat
tool written to support legacy kiwi commandlines. An example
could look like the following call:
sudo kiwi --compat -- --build /my/description --type vmx -d /my/dest
Please be aware the -- is required to tell docopt to treat all
options as parameters