With the recent update to tox 2.0.x, environment variables such as
OS_AUTH_URL are not passed by default, resulting in tests errors
mentionning Keystone authentication failures.
This patch reads credentials from the 'functional_creds.conf' config
file, like it is done in novaclient (and soon in glanceclient and
neutronclient).
Reading credentials the old way (the environment) is still possible.
Change-Id: I2ec1df481aba7a3866fc8dbc912311de02c22d11
Related-Bug: #1455102
dec3d08371