Adding RBAC namespace selection for metrics server

[Rotzke]

Provide a description of what has been changed

Checklist

• I have verified that my change is according to the deprecations & breaking changes policy
• Commits are signed with Developer Certificate of Origin (DCO - learn more)
• README is updated with new configuration values (if applicable) learn more
• A PR is opened to update KEDA core (repo) (if applicable, ie. when deployment manifests are modified)

Fixes #645

[Rotzke]

Hi @JorTurFer ! I have made a duplicate of #657 because it is stuck since the 28. of June and I was actually the one who requested the feature, please review my MR, I excluded version changes as you requested in the initial MR.

[JorTurFer]

To make users 100% aware of the goal, we could call the parameter like controlPlaneServiceAccountsNamespace. I know that there are docs where it's explained how to enable the control plane permission segregation that spread the permissions in multiple service account, but I can't find it (it's a control plane argument). If you find it, I think that it's a good idea adding the link

[Rotzke]

To make users 100% aware of the goal, we could call the parameter like controlPlaneServiceAccountsNamespace. I know that there are docs where it's explained how to enable the control plane permission segregation that spread the permissions in multiple service account, but I can't find it (it's a control plane argument). If you find it, I think that it's a good idea adding the link

@JorTurFer I have renamed the variables, haven't really found those docs you mentioned, though it is not a command line argument for any of control plane components, I guess. Can we still get this merged into the next release/minor/patch version? Would be really cool, we are shivering in anticipation, waited for some time. Thanks in advance!

[JorTurFer]

Can we still get this merged into the next release/minor/patch version?

Yeah! just let me look for the link a few days more and definitively this will be included for next version