Add a dox.yml config file

Dox is a new tool similar to tox but on docker. One of the benefits is
that it eliminates the need of installing all the package dependencies,
which removes the burden from developers.

The tool is still under heavy development but since Zaqar's test suite
is clean and organized, it may be a good project for early adoption.

Change-Id: Iec2698448618fee3019a6877c5c2ae87fda677b6
Merge "Add a dox.yml config file"