Feature/support passing ref to user data

[jaekunchoi]

Ref is now supported in the method from_file of helper file.
Also added unit test
Should address issue #724
@markpeek please review. I was able to successfully run userdata with Ref on AWS.

[markpeek]

Nice change. I have two questions:

1. It seems this will always look for Ref. Perhaps it should be an option or another function name search for Ref.
2. Right now it only looks for 1 Ref. Is there a use case to look for multiple Ref's?

[jaekunchoi]

@markpeek thanks for the feedback. I've updated userdata script with multiple refs and asserting these results in the test. It passes with mutliple Refs being replaced.
I've also added an option to include Ref.

Thanks again!

[jaekunchoi]

@markpeek do you have any feedback on the changes?

[ollikaven]

Plans to merge this?