ensure-sphinx: do not attempt a gettext install if exists

Avoid failure due to "become: yes" when sudo is not allowed (in
a container for instance). This patch check if gettext is installed
before calling the package module with become: yes.

Change-Id: Ibb7f65afad234b39e50aba429b633d70938738f5

roles/ensure-sphinx/tasks/main.yaml

@@ -1,9 +1,15 @@
+- name: Check for gettext installed
+  command: bash -c "type gettext"
+  ignore_errors: yes
+  register: gettext_exists
+
 # TODO(mordred) Make this a list of known binary depends that sphinx needs
 - name: Install gettext package
   package:
     name: gettext
     state: present
   become: yes
+  when: gettext_exists.rc != 0
 
 - name: Find Constraints File
   include_role: