Make cert-manager dependency optional

[avishefi]

Requirement - what kind of business use case are you trying to solve?

Remove non-strict dependency on cert-manager, have the option to pass a tls certificate secret name instead of creating Issuer and Certificate and fix the documentation to reflect that.

Proposal - what do you suggest to solve the problem or improve the existing situation?

Make cert-manager kustomize resources optional, and allow disabling them with a value and pass the tls secret name instead which will be used for the webhooks.

The following PR for the jaeger-operator Helm chart already makes the dependency on cert-manager optional by providing a way to disable the creation of Issuer and Certificate and passing a tls certificate secret name instead: jaegertracing/helm-charts#368