Custom certificates do not work when set for both external and TLS listeners at the same time

When user decides to configure custom TLS certificate for both the TLS and external listeners, they will be both mounted into the same path and the pod creation will fail:

create Pod my-cluster-kafka-0 in StatefulSet my-cluster-kafka failed error: Pod "my-cluster-kafka-0" is invalid: spec.containers[0].volumeMounts[7].mountPath: Invalid value: "/opt/kafka/custom-certs": must be unique

They need to use separate directory / paths to mount the files.

Created by Strimzi#2341