Extend automated Test for ofagent restart

Create robot test case to ensure correct behaviour in case of ofagent restart

1. bring up system
2. provision subscriber
3. ensure correct flows/device/onus/ports/dhcp-alloc/aaa-users and voltha state
4. scale down of agent replicas
5. observe NO change in ONOS state or volta state other than device being disconnected and ports in ONOS being in disables state
6. scale up of agent
7. verify that state is identical to point 3 (system should be back up and running with no difference)