Update grpc-robot to refer latest device-management-interface library

While executing the device-management-interface tests (e.g: https://jenkins.opencord.org/view/VOLTHA-2.X-Tests/job/periodic-voltha-test-DMI/503/console), system is generating a warning message: [ WARN ] installed package device-management-interface==1.2.1 not supported by library, using version 1.0.0 On further investigation we found out it is the grpc-robot library (https://github.com/opencord/grpc-robot/tree/master/grpc_robot/services/dmi) that is throwing this warning.
Currently, grpc-robot only supports device-management-interface library versions from 0.9.1 to 1.0.0, though the latest available release version is 1.2.1 that we would like to use here.