Use one KmsIssuer for certificate requests from all namespaces

[gietschess]

Instead of an additional custom resource (e.g. ClusterKmsIssuer) as mentioned in issue #16 i'd like so suggest an option to overwrite the namespace where the manager is looking for the KmsIssuer object instead of using the namespace where the certificate was requested.

New optional argument: "--global-kmsissuer-namespace".

[maruina]

Hey @gietschess, thanks for your PR.
Looking at https://cert-manager.io/docs/concepts/issuer/#namespaces it seems that the "official" way of doing it would be with a ClusterKSMIssuer. Can I ask you:

• why the suggestion about writing the namespace with a flag
• do you have an example of other issuer using this approach instead of a cluster level resource?