More useful info and narrower columns on image list
Unit test for default user data, and bug fix to use empty strings for missing vars, not "null"
Speed up tests, add docs, remove warnings
ASGARD-1079 - Create simple user data for new AMI types

Made a new plugin interface AdvancedUserDataProvider which takes a LaunchContext instead of a three strings, so that we can add to the functionality again later without replacing the interface so many users have implemented.

The DefaultAdvancedUserDataProvider implementation simply delegates to the original three-string UserDataProvider interface, so existing Asgard admins should not notice a difference.

The new NetflixAdvancedUserDataProvider is open source as an example of the type of logic one can put into an AdvancedUserDataProvider. Netflix's Asgard configuration uses this class, but most companies would not want to use this class for anything other than inspiration.
GroovyDoc
Prettier test code with stripIndent
Merge pull request #385 from joesondow/ASGARD-1079-revised-user-data

ASGARD-1079 - Create simple user data for new AMI types