add vnf_id to the service record

[stevenvanrossem]

see discussion here:sonata-nfv/son-gkeeper#504

Its needed to include the vnf_id specified in the NSD, also in the NSR.
The SLM uses the NSR schema to validate the NSR.
Currently, the service record mentions only this:

"network_functions": [
{
"vnfr_id": "xxxxx"
},
{
"vnfr_id": "yyyyy"
}
],
can vnf_id be added?