Cert-manager is not working correctly with multiple installations on one cluster

[rudivanhierden]

Description
When deploying multiple openstad namespaces on a single cluster, cert-manager is currently installed for each namespace. This results in errors and an inability to create certificates.

Suggested fix
The cert-manager should not be installed in the openstad namespace, but in its' own cert-manager namespace. Each openstad namespace should be linked to the cert-manager in the cert-manager namespace through the correct annotations.

Existing clusters will need to be updated to make sure all certificates are handled by the correct cert-manager.

Related ticket: https://trello.com/c/RXPTVxdh/89-kubernetes-cert-manager-verplaatsen-naar-eigen-namespace