allow deletion of workers

[akanso]

The deletion of workers has only been allowed through a scaleStrategy where the specific names of the pods to be removed has to be defined.

This PR keeps that logic, but adds the ability to remove random workers if the scaleStrategy does not define any specific pods.

E.g. if current running worker pods in a group is 5
and you modify the replicas = 3 with only 1 pod in the scaleStrategy, e.g.

scaleStrategy:
     workersToDelete:
     - raycluster-complete-worker-small-group-bdtwh

Then the second to be removed will be randomly selected by the ray operator