Implement namespace support for exceptions

[mblaschke]

Add namespace support for exceptions for shared clusters.

[CLAassistant]

All committers have signed the CLA.

[rbren]

Great work, and thanks for adding a test! Just one little note

[rbren]

Can you also add documentation here: https://github.com/FairwindsOps/polaris/blob/master/docs/usage.md#exemptions

[mblaschke]

Done

[marcemv90]

With this new feature is it possible to ignore completely a namespace? Because I would like to get rid of the kube-system namespace in the dashboard.

[baderbuddy]

This looks really good to me! Thank you!

[baderbuddy]

@marcemv90 this should allow all controllers within a namespace to be exempted (since it matches on prefix you could add a controller-name of "") but you'd have to specify the rules to exempt still