vagrant: Update box to fedora/26-cloud-base
vagrant: Update box to fedora/26-cloud-base

The vagrant user has the uid 1000 now (it had 1001 before), so we cannot
use 1000 for the admin user anymore. Remove `-u 1000` from the
adduser(8) call. It shouldn't matter which uid the admin user has.

Closes #7425
Reviewed-by: Stef Walter <8a7fbf0f15d3ad982af54ef28f5c117d2494b76c@redhat.com>

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