Allow full kustomisation of aad-msi-binding.yaml

[brk3]

Currently the name of the AzureIdentity / AzureIdentityBinding objects
created as part of the aad-pod-identity lz addon have hardcoded names.
This made it not possible to create more than one pair for one MSI.

This patch applies kustomise to each field allowing for more
customisation.

It also adds a new 'selector' argument in the case the user doesn't want
the name of the MSI used in this field:

managed_identities = {
  ingress_msi = {
    lz_key = "aks"
    aadpodidentity_selector = "ingress"
    msi_keys = [
      "ingress",
    ]
  }
}

If aadpodidentity_selector is not specified the MSI name is used as
before.

PR Checklist

---

• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My code follows the code style of this project.
• I ran lint checks locally prior to submission.
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Description

Does this introduce a breaking change

• YES
• NO

Testing

[hieumoscow]

LGTM, thanks @brk3
Before we did allow to specify selector value but tfvar was quite verbose. I like this approach as msi name might contains weird characters that breaks the creation of azureidentity & azureidentitybinding. Cheers