Default to "omit" mode from directory creation

To avoid a situation where the mode is set on an NFS mount that has
different permissions to 755, we should omit the mode by default.

The default value on creation is 755 in anycase, so if the directory
does not exist it will be created with 755 as before.

Change-Id: I926f46ba2e8a8ce84b4f127dbeabb0daab0b3951
Closes-Bug: #1680233

tasks/glance_pre_install.yml

@@ -35,7 +35,7 @@
     state: directory
     owner: "{{ item.owner|default(glance_system_user_name) }}"
     group: "{{ item.group|default(glance_system_group_name) }}"
-    mode: "{{ item.mode|default('0755') }}"
+    mode: "{{ item.mode|default(omit) }}"
   with_items:
     - { path: "/openstack", mode: "0755", owner: "root", group: "root" }
     - { path: "/etc/glance", mode: "0750" }