Updated DISCOVERY_OAUTH_URL_ROOT

Using EDXAPP_LMS_ROOT_URL instead of EDXAPP_LMS_ISSUER. This removes the need to define another EDXAPP_* override in our secure variables.

playbooks/roles/discovery/defaults/main.yml

@@ -54,7 +54,7 @@ DISCOVERY_CACHES:
 DISCOVERY_VERSION: "master"
 DISCOVERY_DJANGO_SETTINGS_MODULE: "course_discovery.settings.production"
 DISCOVERY_URL_ROOT: 'http://discovery:18381'
-DISCOVERY_OAUTH_URL_ROOT: '{{ EDXAPP_LMS_ISSUER | default("http://127.0.0.1:8000/oauth2") }}'
+DISCOVERY_OAUTH_URL_ROOT: '{{ EDXAPP_LMS_ROOT_URL | default("http://127.0.0.1:8000") }}/oauth2'
 
 DISCOVERY_EDX_DRF_EXTENSIONS:
   OAUTH2_USER_INFO_URL: '{{ DISCOVERY_OAUTH_URL_ROOT }}/user_info'