openonuAdapterGo: test if ONU Model is correctly displayed via voltctl

A new test case (e.g. in "Voltha_ONUStateTests.robot") should be implemented to test if the ONU Model is correctly displayed via voltctl after the ONU has been brought up. Serial Number, Vendor and Vendor-ID should be checked additionally.

{{# show device details
voltctl device list -f Type=brcm_openomci_onu --format 'table{{.Id}}\t{{.SerialNumber}}\t{{.Vendor}}\t{{.VendorId}}\t{{.Model}}'

1. output with BBSIM-ONU
   ID SERIALNUMBER VENDOR VENDORID MODEL
   4f0c6485-c120-4de8-bac4-11be8657dfa0 BBSM000a0001 BBSM BBSM v0.0.1}}