Install rally only when rally_install is set

Currently we install rally regardless of user will.
It is default behaviour which is not possible to override.
So we add rally_install variable which defaults to false to control
whether to proceed with rally installation or not.

Change-Id: I325dee6ec071b6675bcfdadaf884ce8c26cf1ae9

playbooks/os-rally-install.yml

@@ -30,6 +30,10 @@
   tags:
     - rally
   pre_tasks:
+    - name: End playbook
+      meta: end_play
+      when:
+        - (not (rally_install | default(false)) | bool)
     - include_tasks: common-tasks/unbound-clients.yml
       when:
         - hostvars['localhost']['resolvconf_enabled'] | bool

tests/roles/bootstrap-host/templates/user_variables.aio.yml.j2

@@ -243,6 +243,7 @@ lxc_container_bind_mounts:
 ## Always setup tempest, the resources for it, then execute tests
 tempest_install: yes
 tempest_run: yes
+rally_install: yes
 
 # Do a gateway ping test once the tempest role creates it
 tempest_network_ping_gateway: yes