CI: clean up requirements installation

We install kolla-ansible requirements in Zuul's Ansible playbooks.
This patch cleans up the installation in scripts so that they are
only concerned with auxiliary requirements:
- ansible (since we do not track it in requirements)
- ara (for log summaries)
- openstack clients (for first init and tests after deployment)

Additionally this patch installs openstack clients in a separate
virtualenv.
Note that all kolla-ansible requirements, ansible and ara are still
installed system-wide.

Change-Id: Iac04082ad39a9d823c515ba11c5db9af50ed225f
Signed-off-by: Radosław Piliszek <radoslaw.piliszek@gmail.com>