fix xds credentials being silently ignored

[erikjoh]

Fixes #565

Before change:

GRPC_XDS_BOOTSTRAP=/tmp/b.json go run cmd/grpcurl/grpcurl.go xds:///<target service> list
Failed to dial target host "xds:///<target service>": tls: failed to verify certificate: x509: certificate is not valid for any names, but wanted to match <target service>
exit status 1

After change:

GRPC_XDS_BOOTSTRAP=/tmp/b.json go run cmd/grpcurl/grpcurl.go xds:///<target service> list
grpc.health.v1.Health
grpc.reflection.v1.ServerReflection
grpc.reflection.v1alpha.ServerReflection
...

[erikjoh]

@bcleenders I assume you also would be interested in this 😁