Add default timeout for snapshot create

[optik-aper]

Description

Add a default timeout on the create for snapshot resources. This allows using a timeouts block to set a custom value, ie:

resource "vultr_snapshot" "sn" {
  instance_id = resource.vultr_instance.inst.id
  description = "terraform timeout test"

  timeouts {
    create = "60m"
  }
}

This will also increase the default from 20 minutes to 30 minutes without need for customization.

Related Issues

Resolves #509

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?