fix: retrieve configuration resources by labels

[fra98]

Description

This PR fixes a bug in Liqoctl where Configuration resources were retrieved by name instead of labels.

It also makes private the functions that forge default names on Kubeconfig, Nonce and SignedNonce resources.

[adamjensenbot]

Hi @fra98. Thanks for your PR!

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Make sure this PR appears in the liqo changelog, adding one of the following labels:

• kind/breaking: 💥 Breaking Change
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