Fix default tag checking for vultr_kubernetes resource

[timurbabs]

Description

The current code iterates over the node pools but immediately returns an error if the first node pool does not have the tag tf-vke-default. This means it does not check the remaining node pools for the required tag. As a result, if the node pool with the tf-vke-default tag is not the first in the list, the function erroneously returns an error like:

Error: unable to set resource kubernetes default node pool with tag tf-vke-default for <cluster_id>. 
You must set the default tag on one node pool before importing.

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you linted your code locally prior to submission?
• Have you successfully ran tests with your changes locally?