Dependency var_files

[yorinasub17]

This implements support for specifying var_files on a dependency so that you can import all the vars written out to a var file when invoking the dependency. The use case for this is that this can make it possible for customers to write a top level boilerplate.yml configuration that encodes both:

• the template to invoke
• the variable values to invoke with

This is most useful in the Reference Architecture deployment, where we can provide customers with a repeatable configuration artifact that lets them recreate the whole ref arch code. Not to mention the nice side effect of providing them with the version that was used to render everything.

[yorinasub17]

NOTE: The dependencies-remote test is expected to fail, because it points to master and the updated text files are only in this branch.

[yorinasub17]

Thanks for review! Going to merge this in now. Won't cut a release until I confirm the tests are passing.