Add open exchage rates API key

CHANGELOG.md

+- Role: discovery
+  - Added `OPENEXCHANGERATES_API_KEY` for retrieving currency exchange rates.
+
 - Role: edxapp
   - Added `ENTERPRISE_SUPPORT_URL` variable used by the LMS.
 

playbooks/roles/discovery/defaults/main.yml

@@ -96,6 +96,8 @@ DISCOVERY_EMAIL_HOST_PASSWORD: ''
 
 DISCOVERY_PUBLISHER_FROM_EMAIL: !!null
 
+DISCOVERY_OPENEXCHANGERATES_API_KEY: ''
+
 DISCOVERY_GUNICORN_EXTRA: ''
 
 DISCOVERY_EXTRA_APPS: []
@@ -123,5 +125,7 @@ discovery_service_config_overrides:
 
   PUBLISHER_FROM_EMAIL: '{{ DISCOVERY_PUBLISHER_FROM_EMAIL }}'
 
+  OPENEXCHANGERATES_API_KEY: '{{ DISCOVERY_OPENEXCHANGERATES_API_KEY }}'
+
 # See edx_django_service_automated_users for an example of what this should be
 DISCOVERY_AUTOMATED_USERS: {}