When packer validate is invoked, it does not try to evaluate the
datasources before attempting to decide if the template is valid.
In many cases, this works, but sometimes it will fail as the value is
unknown by the validation code.
Since the validation code for all the elements of a Packer template is
left to be implemented by plugins, we cannot rely on checking for
unknown values everywhere, especially since the unknown references are
replaced automatically by a value of the right type for the
configuration expected.
So, in order for such configurations to be validable, we add an extra
option to packer validate, that will let users evaluate the datasources
from a template.
Lucas Bajolet
bb2384c56a