Workaround to install specific version of dogpile.cache

openstacksdk recently made changes to avoid newer dogpile.cache
versions[1], and this breaks while loading plugins from rally-openstack.
As a workaround, temporarily installing a dogpile.cache that satisfies
openstacksdk's requirements.

[1] openstack/openstacksdk@fd61b54

Change-Id: I7cb41b7c484f5afd180b0d815dd655459382fba3

Author: aakarshg

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

@@ -15,6 +15,12 @@
     line: 'export REQUESTS_CA_BUNDLE={{ overcloud_ca_path }}'
   when: overcloud_ca_path is defined
 
+- name: Temporary workaround to avoid conflicts with dogpile.cache
+  pip:
+    name: dogpile.cache
+    version: 0.6.8
+    virtualenv: "{{ rally_venv }}"
+
 - name: Install Rally with OpenStack plugins into rally-venv
   pip:
     name: rally-openstack