Add validation for read only fields

[avisiedo]

As an Administrator I want some CRD attributes to be read-only that meet with:

• When the CRD is created, the read-only attributes can not be modified unless the CRD is deleted and created again.
• An admission Webhook will manage the read-only fields.

Additional information:

• Admission Webhooks.
• Dynamic Admission Control.
• Admission-plug-ing - Admission webhooks.

[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: avisiedo, frasertweedale

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/freeipa/freeipa-operator/blob/master/OWNERS)~~ [avisiedo,frasertweedale] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[openshift-ci[bot]]

New changes are detected. LGTM label has been removed.

[avisiedo]

I have made minor changes:

• Cleanup a little the changes.
• Keep cert-manager rules only at mk/cert-manager.mk file.
• Add some comments to the code.
• Update README.md doc.