Upgrade td-grpc-bootstrap to v0.19.0 (#242)

Author: ejona86

config/common.cfg

@@ -1,5 +1,5 @@
 --resource_prefix=psm-interop
---td_bootstrap_image=us-docker.pkg.dev/grpc-testing/trafficdirector/td-grpc-bootstrap:ee0d85b47aad4b6352975505996116de838843ad
+--td_bootstrap_image=us-docker.pkg.dev/grpc-testing/trafficdirector/td-grpc-bootstrap:a756beb5f56c84555095aa3622bdddd843fef95a
 
 # The canonical implementation of the xDS test server.
 # Can be used in tests where language-specific xDS test server does not exist,