adding the www-data user in case it doesn't exist

playbooks/roles/common/tasks/main.yml

 ---
 - include: create_users.yml
 
+- name: Add user www-data
+  # This user should be created on the system by default
+  user: name=www-data
+  tags:
+  - pre_install
+  - update
+
 - name: Create application root
   # In the future consider making group edx r/t adm
   file: path={{ app_base_dir }} state=directory owner=root group=adm mode=2775