respect vpc2 order for resource_vultr_instance

[quantimnot]

Description

The vpc2_ids were stored in a set type, so the order wasn't preserved. I changed it to a list type.

NOTE: This is an incomplete change. A check for duplicate values should be added. This change should be done to other resources like bare metal.

NOTE: I am not a Go programmer.

NOTE: I am not doing anymore with this PR. You can merge it, or close it.

Related Issues

Fixes #550

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?