Add default value for log_dirs in os-log-dir-setup.yml

There is no default value for log_dirs in the os-log-dir-setup.yml
which means that any role that includes it without that value will
crash and burn.  We set a default here to protect us from breaking
roles in the future if we forget to specifiy log_dirs on baremetal.

Change-Id: I07bf9f4bce7bb53ecc305d260142e878797d6c04
Closes-Bug: #1761781

playbooks/common-tasks/os-log-dir-setup.yml

@@ -27,7 +27,7 @@
   file:
     path: "{{ item.src }}"
     state: directory
-  with_items: "{{ log_dirs }}"
+  with_items: "{{ log_dirs|default([]) }}"
   when: is_metal | bool
   tags:
     - common-log
@@ -40,7 +40,7 @@
     group: "{{ item.group|default(omit) }}"
     state: "link"
     force: "yes"
-  with_items: "{{ log_dirs }}"
+  with_items: "{{ log_dirs|default([]) }}"
   when: is_metal | bool
   tags:
     - common-log