fix: rke2 registration token generated twice

[salasberryfin]

What this PR does / why we need it:

The RKE2 Config controller is creating the registration token twice (intermittently) and updating the secret that contains the token so different nodes are trying to get registered with different tokens and failing to do so.

We must ensure this secret is created once and its value remains unedited so that nodes can be registered to the cluster.

This initial proposal suggests detecting if the secret exists and skip token creation if it does.

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 #233

Special notes for your reviewer:

We should add test cases that cover the logic of this controller: ##235 tracks this follow-up issue.

Checklist:

• squashed commits into logical changes
• includes documentation
• adds unit tests
• adds or updates e2e tests

[salasberryfin]

PR is now updated based on review feedback. Current logic will:

• Try to create secret and store registration token.
• If secret already exists, retrieve registration token value.

[Danil-Grigorev]

lgtm

[richardcase]

Tested this locally and i didn't see the same duplicate token creation.