Drop / enable explicitly cert-manager dep in operator helm chart

[pschiffe]

Currently there's a hard dep on cert-manager helm chart when operator is instructed to use cert-manager's CRD.

Cert-manager is widely used and usually already present in the cluster, installed according to the needs of the cluster. Also the cert-manager version specified in https://github.com/OT-CONTAINER-KIT/redis-operator/blob/master/charts/redis-operator/Chart.yaml#L25 is outdated.

It would be great if the redis operator could create cert-manager certificate without installing cert-manager. For example by either dropping cert-manager dep from Chart.yaml or using different condition variable, such as certmanager.install or similar.

Related to #784

redis-operator version: 0.19.0