This adds a new resource to template to generate multipart cloudinit
configurations to be used with other providers/resources.
The resource has the ability gzip and base64 encode the parts.
Building on the work of #3846, deprecate `filename` in favor of a
`template` attribute that accepts file contents instead of a path.
Required a bit of work in the interpolation code to prevent Terraform
from assuming that template interpolations were resource variables that
needed to be resolved. Leaving them as "Unknown Variables" prevents
interpolation from happening early and lets the `template_file` resource
do its thing.
