Change default mode of mount files to 644

In case of setting file permissions to 640 systemd will raise an
alert [1]. In order to avoid spamming in logs, we change default mode to
644 as no passwords or credentials are going to be stored in this file.

With that we provide a way to override file mode if needed.

[1] https://paste.openstack.org/show/b83tSrjni8iB2WNsvDfQ/

Change-Id: I73164e9d4b02d7b3fae21b912369afcd1cc64abf

defaults/main.yml

@@ -54,6 +54,7 @@ systemd_default_mount_options: 'defaults'
 #     Where: "/var/lib/glance/images"
 #     type: "nfs"
 #     options: "_netdev,auto"
+#     unit_mode: "0640"
 #   - what: "/openstack/swap.img"
 #     priority: "0"
 #     options: "%%"

tasks/systemd_mounts.yml

@@ -56,7 +56,7 @@
     dest: "/etc/systemd/system/{{ mount_service_name.stdout }}"
     owner: "root"
     group: "root"
-    mode: "0640"
+    mode: "{{ item.unit_mode | default('0644') }}"
     config_overrides: "{{ item.config_overrides | default({}) }}"
     config_type: "ini"
   when: