Rewrite YamlParser to improve error messages and JJB yaml extensibility

The goal of this blueprint is to articulate the pressing need for a rewrite of YamlParser in jenkins-job-builder.

I have attempted to improve defaults handling without much success due to the strange and difficult-to-grok, organically-grown data structure traversal within exandYaml. With a better-conceived yaml parsing class such improvements will become easier to make.

Error messages (for example when a JJB variable is missing) are almost impossible to understand because of all the data munging going on at different points within YamlParser. With a yaml parsing class that attachs information such as the filename where each piece of data is obtained we can at least point users to the relevant files containing the tidbit that is failing to be expanded or interpolated.

In addition to that, it is obvious that functionality has creeped into YamlParser that does not really fit the role of "yaml parsing". I was very surprised, for example, to find out that XML generation itself occurs within YamlParser... Also see the somewhat grotesque inclusion of a ModuleRegistry object in YamlParser's attributes.