Reuse existing tempest workspace

If the tempest role is re-run against a host it will fail attempting to
re-initialize over an existing workspace. Update the changed_when and
failed_when conditions to allow reusing an existing workspace. Also
provide the path to the workspace rather than working from within the
tempest directory to avoid stderr and stdout being written to logs.

Change-Id: Ie88cb72b0d7cd3ef07ab269f2580485a7eeae3c6

tasks/tempest_post_install.yml

@@ -76,12 +76,19 @@
 
 - name: Initialise tempest workspace
   shell: |
-    . {{ tempest_venv_bin }}/activate
-    tempest init workspace
+    if [ ! -d {{ tempest_venv_bin | dirname }}/workspace ]
+    then
+      . {{ tempest_venv_bin }}/activate
+      tempest init {{ tempest_venv_bin | dirname }}/workspace
+      exit 3
+    fi
   args:
-    chdir: "{{ tempest_venv_bin | dirname }}"
     executable: /bin/bash
-  changed_when: false
+  register: tempest_init_workspace
+  changed_when: tempest_init_workspace.rc == 3
+  failed_when:
+    - tempest_init_workspace.rc != 0
+    - tempest_init_workspace.rc != 3
   tags:
     # don't trigger ANSIBLE0013
     - skip_ansible_lint