Provide empty default for cinder_backends

In the 'Create nfs shares export file' task, default cinder_backends to
an empty dict to avoid fatal errors when looping through an undefined
var.

Change-Id: I8dc9d61fba42bd58d5d84dd3293e95a6ff4ab768

tasks/cinder_post_install.yml

@@ -67,7 +67,7 @@
   template:
     src: nfs_shares.j2
     dest: "{{ item.value.nfs_shares_config }}"
-  with_dict: "{{ cinder_backends }}"
+  with_dict: "{{ cinder_backends | default({}) }}"
   when:
     - item.value.nfs_shares_config is defined
     - inventory_hostname in groups['cinder_volume']