[improvement] Fix DNS_UNIQUE_ID in templating so it's not required to be set for DNS flavor

[AshleyDumaine]

What this PR does / why we need it: We allow this field to be empty so CAPL can create a unique identifier automatically:

  dnsUniqueIdentifier	<string>
    dnsUniqueIdentifier is the unique identifier for the DNS. This let clusters
    with the same name have unique
    DNS record
    Ignored if the LoadBalancerType is set to anything other than dns
    If not set, CAPL will create a unique identifier for you

However if this var isn't set during template rendering we get an error:

(⎈|kind-tilt:N/A) ~/cluster-api-provider-linode   main  clusterctl generate cluster test-cluster --kubernetes-version v1.31.8 --infrastructure local-linode:v0.0.0 --flavor kubeadm-dns-loadbalancing> test-cluster.yaml
Error: value for variables [DNS_UNIQUE_ID] is not set. Please set the value using os environment variables or the clusterctl config file

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.97%. Comparing base (148daa3) to head (6e86fb3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #883   +/-   ##
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  Coverage   65.97%   65.97%           
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  Files          71       71           
  Lines        7959     7959           
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  Hits         5251     5251           
  Misses       2406     2406           
  Partials      302      302           

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