Set default for GIT_REPO_EXPORT_DIR

Setting GIT_REPO_EXPORT_DIR to default to `/edx/var/edxapp/export_course_repos`

Thanks Carson Gee. https://groups.google.com/forum/#!topic/edx-code/nBfcPQy_W0k

playbooks/roles/edxapp/defaults/main.yml

@@ -121,6 +121,7 @@ EDXAPP_MODULESTORE_MAPPINGS:
     'preview\.': 'draft-preferred'
 
 EDXAPP_GIT_REPO_DIR: '/edx/var/edxapp/course_repos'
+EDXAPP_GIT_REPO_EXPORT_DIR: '/edx/var/edxapp/export_course_repos'
 
 EDXAPP_TENDER_DOMAIN: "None"