Leverage on pbr (Python Build Reasonableness)

Leverage on pbr (Python Build Reasonableness)

pbr can inject requirements into the install_requires, tests_require
and/or dependency_links arguments to setup. They were previously
falsy unset.

It also supports conditional dependencies which can be added to the
requirements (e.g. subprocess32; python_version=='2.7').
Then requirements.py3.txt is removed.

It also removes functest/cli/setup.py which seems false and obsolete.

Dockerfile is updated as well. One temporarily link is created because
of several hardcoded paths in Functest.

Change-Id: I14b3b25e07fbac490dd1d5ce943ec02c5b11f242
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>