If TargetRef Deployment is deleted, we should stop watching it in Controller.

[ramantehlan]

Bug Description

Users point to a TargetRef in Elasti CRD. If this target is deleted, Elasti continues to watch it in Operator. We can handle such scenarios in the operator.

Steps to Reproduce

1. Elasti is already installed, and a ElastiService CRD exists for a target.
2. Delete the target.
3. You will notice that Elasti Operator still has the watch on the deployment/target, which is throwing error now.

Expected Behavior

Identify if the target exists, if not, stop the watch. If the target is recreated, start the watch.

Actual Behavior

If target is deleted in middle, we don't handle it.