Merge project pipeline definitions

It's important to have legacy files for legacy jobs, project-templates
and a legacy dir for legacy playbooks. HOWEVER - the project pipeline
configs are the project pipeline configs. Rather than emitting
99legacy-projects -if there is already a zuul.d/projects.yaml, assume
that it contains a list of projects, read it in, and then merge the
generated project-pipeline definitions with the existing ones. This will
give us a unified project-pipeline definition list. (It loses the
comments in the original file, but there were only a couple that can be
added back by hand.

Change-Id: Ia2f440df47aac0a5b1e6baae3c081bf69d7add03
1 file changed