Adds readinessProbe and livenessProbe to prometheus-adapter jsonnet

Problem: Currently the prometheus-adapter pods are restarted at the same
time even though the deployment is configured with strategy RollingUpdate.
This happens because the kubelet does not know when the prometheus-adapter
pods are ready to start receiving requests.

Solution: Add both readinessProbe and livenessProbe to the
prometheus-adapter, this way the kubelet will know when either the pod
stoped working and should be restarted or simply when it ready to start
receiving requests.

Issue: https://bugzilla.redhat.com/show_bug.cgi?id=2048333