Use the namespace stackable-operators in the SecretClass tls in the OLM package

Currently the namespace default is set which means that the secret secret-provisioner-tls-ca is generated in the default namespace.

Installing via OLM requires the operators to be installed in the namespace stackable-operators because the ClusterRoleBinding of the secret-operator points to this namespace. Therefore, it is consequent to keep things together and also create the secret secret-provisioner-tls-ca in this namespace.

This is a breaking change which must be announced prominently. If pods would use different root CAs at the same time, they cannot communicate with each other anymore.

The Helm installation does not have a requirement on the namespace. It would be possible to also set the namespace in the SecretClass where the operator is installed to, but if the secret-operator will be installed in another namespace then running products would break. So we decided, to keep the default namespace for the Helm installation.