Point to the base box on Vagrant Cloud + updated README

[ringods]

If you point to the published box on Vagrant Cloud, it is no longer needed to create/build the base box locally. With a correct pointer to the base box, the "Install base box" is obsolete. This gives a better user experience to use this setup.

BTW, thanks for your work on this. This helps me in testing my pull request for the Terraform Gitlab Provider locally as Gitlab in a container doesn't run correctly for me due to this errror.