User-Data provider plugin

This is a plugin to satisfy the issue I raised: https://github.com/Netflix/asgard/issues/387

It implements the AdvancedUserDataProvider interface to provide User-Data by using Groovy templates pulled via GETs from configured repos. It's been tested against github, but should work against any http file store.

Please forgive the messy diffs. My gitfoo is low and I made a bit of a mess here... if you like this code but want a better PR, let me know.