Add constraints for _greeter._greet.parameters._count

Change-Id: Iaae4ef15fe535bf7ab50910414e86bb15f91ea81
Reviewed-on: https://weave-review.googlesource.com/1262
Reviewed-by: Vitaly Buka <vitalybuka@google.com>
Reviewed-by: Alex Vakulenko <avakulenko@google.com>