Add platform description variable to edxapp.

CHANGELOG.md

 - Role: edxapp
   - Added `EDXAPP_ENTERPRISE_TAGLINE` for customized header taglines for different enterprises.
+  - Added `EDXAPP_PLATFORM_DESCRIPTION` used to describe the specific Open edX platform.
 
 - Role: edxapp
   - Added `ENTERPRISE_SUPPORT_URL` variable used by the LMS.

playbooks/roles/edxapp/defaults/main.yml

@@ -161,6 +161,7 @@ EDXAPP_PARTNER_SUPPORT_EMAIL: ''
 EDXAPP_AUDIT_CERT_CUTOFF_DATE: null
 
 EDXAPP_PLATFORM_NAME: 'Your Platform Name Here'
+EDXAPP_PLATFORM_DESCRIPTION: 'Your Platform Description Here'
 EDXAPP_STUDIO_NAME: 'Studio'
 EDXAPP_STUDIO_SHORT_NAME: 'Studio'
 EDXAPP_ANALYTICS_DASHBOARD_NAME: "{{ EDXAPP_PLATFORM_NAME }} Insights"
@@ -985,6 +986,7 @@ generic_env_config:  &edxapp_generic_env
   LMS_ROOT_URL: "{{ EDXAPP_LMS_ROOT_URL }}"
   PARTNER_SUPPORT_EMAIL: "{{ EDXAPP_PARTNER_SUPPORT_EMAIL }}"
   PLATFORM_NAME: "{{ EDXAPP_PLATFORM_NAME }}"
+  PLATFORM_DESCRIPTION: "{{ EDXAPP_PLATFORM_DESCRIPTION }}"
   ANALYTICS_DASHBOARD_NAME: "{{ EDXAPP_ANALYTICS_DASHBOARD_NAME }}"
   STUDIO_NAME: "{{ EDXAPP_STUDIO_NAME }}"
   STUDIO_SHORT_NAME: "{{ EDXAPP_STUDIO_SHORT_NAME }}"