rfc2136: Add cli flag for Kerberos realm and improve tutorial

[stovemeerkat]

Description

This adds a new optional cli flag --rfc2136-kerberos-realm for the rfc2136 provider.
It also updates the rfc2136 tutorial to mention this flag and adds some other tips based on my own experience with setting up external-dns.

Checklist

• Unit tests updated
• End user documentation updated

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[stovemeerkat]

/assign @njuettner

[seanmalloy]

/lgtm
/kind feature
/assign @Raffo

[Raffo]

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Raffo, seanmalloy, stovemeerkat

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