Pin python-novaclient to 17.6.0

With[1] included in novaclient-18.3.0 use of
floating and other keyword args do not work.
Until browbeat code base is compatible with
novaclient-18.3.0, let's pin it.

It is follow up of [2] as that was not sufficient.

[1] https://github.com/openstack/python-novaclient/commit/80ee69a
[2] https://review.opendev.org/c/x/browbeat/+/874987

Closes-Bug: #2008235
Change-Id: I9d89c15cd2b67d7801ed197caa40d37f1901612c

ansible/install/roles/rally/tasks/main.yml

@@ -54,6 +54,13 @@
     name: 'SQLAlchemy<2.0.0'
     virtualenv: "{{ rally_venv }}"
 
+# Till browbeat code is compatible with novaclient-18.3.0
+# https://bugs.launchpad.net/openstack-browbeat/+bug/2008235
+- name: Ensure python-novaclient<=17.6.0
+  pip:
+    name: 'python-novaclient<=17.6.0'
+    virtualenv: "{{ rally_venv }}"
+
 - name: Create rally configuration directory
   file:
     path: "{{ rally_venv }}/etc/rally"