With the defined check, it's possible to have config syntax errors

Ansible will then skip the task and not write a config file.
I content that we want ansible to bail out if the dict it gets
in edx_service_config is invalid.

I'll need to change the two roles that pass no config (xqwatcher
and harstorage) to pass {} which will evaluate to false and skip
making a config.

Noticed this when manually testing config changes and saw missing
files rather than syntax errors.  In particular, this will make
our config checking code much better.

playbooks/roles/edx_service/tasks/main.yml

@@ -95,7 +95,7 @@
     src: "config.yml.j2"
     dest: "{{ COMMON_CFG_DIR  }}/{{ edx_service_name }}.yml"
     mode: "0644"
-  when: edx_service_config is defined
+  when: edx_service_config
   tags:
     - install
     - install:configuration