Remove None/null config settings from the yaml config

This means that your default in app/settings/base.py will retain the
default if there's no default set here.  Otherwise it would be set to
None instead.

OPS-2714

playbooks/roles/edx_service/defaults/main.yml

@@ -23,6 +23,10 @@ edx_service_repos: []
 # and not write out a config at all.
 edx_service_config: {}
 
+# If you would like edx_service to strip out !!null settings before writing out
+# the yaml config, set this to true.
+edx_service_config_filter_nones: false
+
 #
 # OS packages
 #

playbooks/roles/edx_service/templates/config.yml.j2

 ---
 # {{ ansible_managed }}
 
+{% if edx_service_config_filter_nones -%}
+  {% for key, value in edx_service_config.copy().iteritems() -%}
+    {% if value is none -%}
+      {% do edx_service_config.pop(key) %}
+    {%- endif %}
+  {%- endfor %}
+{%- endif %}
+
 {{ edx_service_config | to_nice_yaml }}

playbooks/roles/xqueue/meta/main.yml

@@ -5,6 +5,7 @@ dependencies:
   - role: edx_service
     edx_service_name: "{{ xqueue_service_name }}"
     edx_service_config: "{{ XQUEUE_CONFIG }}"
+    edx_service_config_filter_nones: true
     edx_service_repos: "{{ XQUEUE_REPOS }}"
     edx_service_user: "{{ xqueue_user }}"
     edx_service_home: "{{ xqueue_home }}"