Feature/remove hdp version attrs

[dereklwood]

This removes the duplication of the hdp "build number" attributes, and instead only ever gets the build number via the hdp_version helper library.

• in the attributes, it uses a ruby %{} delayed interpolation token
• in the default recipe, it interpolates it, with a value of a call to the hdp_version helper library
• added a test for it

[wolf31o2]

This is great. That was one pain point that made the cookbook ugly.

[dereklwood]

:)