In order to prepare for implementing requirements management by the
OpenStack requirements management process, and to improve the
reliability and effectiveness of test execution, this patch implements
some changes to the tox configuration:
- The minimum tox version is increased in order to be able to use
 constraints for the python packages.
- The OpenStack upper-constraints are used when preparing the test
 venv for the linters checks.
- Any proxy environment variables set on the test host are passed
 into the venv to enable testing from behind a proxy.
- The environment variables used by Ansible tests are moved into
 a new venv called 'ansible' and this environment is inherited
 by all Ansible-related tests.
- The docs test will clean-up an existing build directory before
 executing the docs build.
- The releasenotes build cannot use upper-constraints at this point,
 so it doesn't.
- The Ansible role download will no longer ignore errors so that any
 problems discovered will result in a failed test.
- The human readable logging callback plugin is implemented for
 functional testing.
- The ansible test requirements are moved into tox.ini to ensure
 compliance for requirements.txt/test-requirements.txt for the
 global-requirements management contract.
- The ~/.ansible directory as a whole is not deleted. Instead only
 the plugins and roles folders are deleted to ensure that zuul's
 Ansible artifacts are left in-place.
- The ansible-lint version is updated to support execution against a
 folder, and the test now executes against the entire role to ensure
 that it captures all applicable files for lint testing.
This is a combined port of the following:
- https://review.openstack.org/323507
- https://review.openstack.org/338193
- https://review.openstack.org/332443
- https://review.openstack.org/338193
- https://review.openstack.org/339493
Change-Id: Ie08d1c8c82220e496bfa6d8b6743b5b3318c72ed