ensure-tox: Avoid Reinstalling tox venv if present

On a static node where tox isn't present in the PATH, tox would be always
reinstalled even if already present in venv because it's not checked.

This patch adds tox_venv check, the first matching line is used to get
executable path

Change-Id: Ia27af308cd3b5e2ce5efe36508fe82468e88cbc7

roles/ensure-tox/tasks/main.yaml

@@ -6,7 +6,7 @@
 
 - name: Check if tox is installed
   shell: |
-    command -v {{ tox_executable }} || exit 1
+    command -v {{ tox_executable }} {{ tox_venv_path }}/bin/tox || exit 1
   args:
     executable: /bin/bash
   register: tox_preinstalled
@@ -14,7 +14,7 @@
 
 - name: Export preinstalled tox_exectuable
   set_fact:
-    tox_executable: '{{ tox_preinstalled.stdout }}'
+    tox_executable: '{{ tox_preinstalled.stdout_lines[0] }}'
     cacheable: true
   when: tox_preinstalled.rc == 0