service-rate should not always be required

Service create validation always fails if service-rate is not set in the service-create rpc (see ServicehandlerTxRxCheck.java). However, the openroadm service model (see org-openroadm-common-service-types.yang) says:

leaf service-rate {
   when "../service-format != 'OMS' and ../service-format != 'ODU'" {
      description
         "service rate not applicable when service
         format is roadmline or ODU; valid for OTU since service-rate has
         already been supported for wdm layer OTU services (100 for OTU4)";
   }

 So in these cases validation should pass even if service-rate is missing.