Clustering: Add a count field to stress-test RPC in car yang model

Currently the car yang model has a stress-test rpc which allows for only a 'rate' to be given. It would be useful in some cases to give a 'count' of cars as well. This will allow us to create a certain number of cars for testing purposes.
If no count is provided, then the current behavior should be supported. That is, test will keep creating cars, until stop-stress-test rpc is issued.