Manage ansible installations within zuul

As a first step towards supporting multiple ansible versions we need
tooling to manage ansible installations. This moves the installation
of ansible from the requirements.txt into zuul. This is called as a
setup hook to install the ansible versions into
<prefix>/lib/zuul/ansible. Further this tooling abstracts knowledge
that the executor must know in order to actually run the correct
version of ansible.

The actual usage of multiple ansible versions will be done in
follow-ups.

For better maintainability the ansible plugins live in
zuul/ansible/base where plugins can be kept in different versions if
necessary. For each supported ansible version there is a specific
folder that symlinks the according plugins.

Change-Id: I5ce1385245c76818777aa34230786a9dbaf723e5
Depends-On: https://review.openstack.org/623927

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