operator doesn't handle deletion of secrets

Currently operator does not handle the deletion of the cacerts secret in the control plane namespace, leaving the mutating webhook with an invalid/outdated caBundle.

• Deploy operator and control plane
• Observe that operator correctly set the webhook caBundle to be the one in the pre-installed "istio-ca-secret" in the control plane namespace. Both commands below should have the same output:

oc get mutatingwebhookconfiguration istiod-basic-istio-system -o jsonpath='{.webhooks[0].clientConfig.caBundle}' | base64 -d
oc -n istio-system get secrets istio-ca-secret -o jsonpath='{.data.ca-cert\.pem}' | base64 -d

• Create a "cacerts" secret in the control plane namespace:

cd <path/to/istio>/samples/certs
oc -n istio-system create secret generic cacerts  --from-file=ca-cert.pem --from-file=ca-key.pem --from-file=root-cert.pem --from-file=cert-chain.pem

• Observe that operator correctly sets the webHook caBundle to be the one in present in the cacerts secret. Both commands below should have the same output:

oc get mutatingwebhookconfiguration istiod-basic-istio-system -o jsonpath='{.webhooks[0].clientConfig.caBundle}' | base64 -d
oc -n istio-system get secrets cacerts -o jsonpath='{.data.ca-cert\.pem}' | base64 -d

• Now delete this cacerts secret:

oc -n istio-system delete secret cacerts

• Observe that operator does not react to this change
• Observe that webhook still uses the old/invalid caBundle.

It should react and update the webhook caBundle to use the certs present in the default "istio-ca-secret", as it was before the creation of the cacerts secret.