add krb5 realm support

[ba-work]

Description

This adds configurable realm support so that the realm and zone are no longer coupled.
Most of the code was already there, looks like this was a small oversight.

I tested this in a real AD environment with success.

This closes #1981.

Checklist

• Unit tests updated
• End user documentation updated

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[Raffo]

I added a comment on validation.

[ba-work]

looks like this was handled/merged/copied via #2015, however my PR includes more tests and validation so I'm going to leave it open

[ba-work]

/assign @Raffo

[seanmalloy]

/kind feature

[Raffo]

/approve
/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ba-work, Raffo

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