Some time ago the placement-api-ref source structure was changed
(Ia2fd62ae7f401cad34ee7c2b355c9a5ab1c93f6b) to use inc-files istead of
putting all in index.rst, but that change broke placement_api_docs.py.
This change modifies placement_api_docs.py to go through all *.inc files
in placement-api-ref directory and scan for the "rest method" header.
Change-Id: I60150ab51173934f848662499dcf6af18625697e