[ci] Do not transmit plugins dir as cli argument

Rally base job copies custom plugins to ~/.rally/plugins dir. This
directory should be loaded automatically. Specifing the same dir via
cli argument `--plugin-paths` leads to PluginWithSuchNameExists errors.

Also, this patch extends rally-task-simple-job job to check that there
are no issues related to loading plugins from ~/.rally/plugins dir

Change-Id: I38adfe6868112d9f2919ccc08c0d40d0bc6b1d51

.zuul.d/rally-task-simple-job.yaml

@@ -3,4 +3,4 @@
     description: A simple job which checks Rally integration with Devstack in Zuul V3 env.
     parent: rally-task-at-devstack
     vars:
-      rally_task: rally-jobs/simple-cinder.yaml
+      rally_task: rally-jobs/simple-job.yaml

rally-jobs/simple-job.yaml

@@ -18,3 +18,15 @@
             users:
               tenants: 2
               users_per_tenant: 2
+    -
+      title: Test a subtask with only one workload
+      description: Check that a fake plugin is loaded from ~/.rally/plugins dir
+      scenario:
+        FakePlugin.testplugin: {}
+      runner:
+        constant:
+          times: 4
+          concurrency: 4
+      sla:
+        failure_rate:
+          max: 0

tests/ci/playbooks/roles/run-rally-task/tasks/main.yaml

@@ -7,7 +7,7 @@
       export RALLY_PLUGINS_DIR="{{ rally_home_dir }}/plugins"
       export RALLY_EXTRA_DIR="{{ rally_home_dir }}/extra"
 
-      rally --rally-debug --plugin-paths "{{ rally_home_dir }}/plugins" task start --task {{ rally_home_dir }}/task.yaml #--task-args-file {{ rally_home_dir }}/task_args_file.yaml
+      rally --rally-debug task start --task {{ rally_home_dir }}/task.yaml #--task-args-file {{ rally_home_dir }}/task_args_file.yaml
     executable: /bin/bash
   register: command_result
   ignore_errors: True