Add EDX_PLATFORM_REVISION env var for Studio/LMS

This is useful for debug output in edx-platform. We used to have the app
generate this value by doing a call to dealer.git on startup, but that
was slow.

CHANGELOG.md

+- Role: edxapp
+  - Added `EDX_PLATFORM_REVISION` (set from `edx_platform_version`). This is for
+  edx-platform debugging purposes, and replaces calling dealer.git at startup.
+
 - Role: veda_pipeline_worker
   - New role to run all (`deliver, ingest, youtubecallback`) [video pipeline workers](https://github.com/edx/edx-video-pipeline/blob/master/bin/)
 

playbooks/roles/edxapp/defaults/main.yml

@@ -982,6 +982,7 @@ generic_env_config:  &edxapp_generic_env
   CREDENTIALS_PUBLIC_SERVICE_URL: "{{ EDXAPP_CREDENTIALS_PUBLIC_SERVICE_URL }}"
   ECOMMERCE_PUBLIC_URL_ROOT: "{{ EDXAPP_ECOMMERCE_PUBLIC_URL_ROOT }}"
   ECOMMERCE_API_URL: "{{ EDXAPP_ECOMMERCE_API_URL }}"
+  EDX_PLATFORM_REVISION: "{{ edx_platform_version }}"
   ENTERPRISE_API_URL: "{{ EDXAPP_ENTERPRISE_API_URL }}"
   COURSE_CATALOG_API_URL: "{{ EDXAPP_COURSE_CATALOG_API_URL }}"
   COURSE_CATALOG_VISIBILITY_PERMISSION: "{{ EDXAPP_COURSE_CATALOG_VISIBILITY_PERMISSION }}"