Separate XML generation from Yaml parsing/interpreting. The goal here
is to allow different sources to provide data for XML generation,
including external API users writing job definitions in pure Python or
JJB developers who would like to work on alternative Yaml parsing code
since the current YamlParser has very likely reached the limits of
what can be reasonably done with one giant expandYaml procedure.
Change-Id: I9da848acac7e944c0e07286b7399b2e1956a58a5
aaae83c623