add externalStagingServer, which will point to stage.m.o in prod b=396438

tools/release/configs/fx-moz18-staging-bootstrap.cfg

@@ -60,6 +60,8 @@ buildTree       = MozillaTest
 # where QA updates/builds go
 stagingUser     = cltbld
 stagingServer   = staging-build-console.build.mozilla.org
+externalStagingUser     = cltbld
+externalStagingServer   = staging-build-console.build.mozilla.org
 # where beta updates/builds go
 ftpServer       = staging-build-console.build.mozilla.org
 # where release updates/builds go