travis: modify configuration
travis: modify configuration

Migrate to container-based builds and pre-install keystone, as it cannot
be intalled directly with pip form PyPI. Do not use tox for testing, as
it is being invoked from a venv and there is no way to force intsall
keystone inside the tox generated venvs.

IFCA/keystone-voms,alvarolopez/keystone-voms