VAGRANT_PROVIDERS variable. Right now, it defaults to only
vmware_desktop, virtualbox support is to follow at some point.
While here, fix the hashicorp URL: s/vagrant/atlas/, which
was result of a sed(1) replace (and my fault).
Sponsored by: The FreeBSD Foundation
VAGRANT_${VAR} variables extracted from VAGRANT_UPLOAD_CONF.
Set ATLAS_${VAR} to VAGRANT_${VAR} if VAGRANT_UPLOAD_CONF
is set. There is intent to intentionally have separate
variants of configuration entries, but the defaults do not
yet have any reason to be different.
Sponsored by: The FreeBSD Foundation