Remove duplicate validation from CRD & Webhook

[we4tech]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:

• Removes the listener.hostname validation from the Webhook
• Adds the listener.hostname validation pattern to the CRD

This PR removes the duplicate validation rules and keeps one source.

Which issue(s) this PR fixes:

Fixes #868

Does this PR introduce a user-facing change?:

NONE

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[k8s-ci-robot]

Hi @we4tech. Thanks for your PR.

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[hbagdi]

/ok-to-test

[robscott]

Thanks for the work on this @we4tech! Mostly LGTM, just 1 tiny nit.

[we4tech]

Hi @robscott, I just addressed your comment. Appreciate your review. Let me know if you find anything else.

[robscott]

Thanks!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: robscott, we4tech

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