Use YAML text instead of JSON in HTTP body

 * Use YAML text instead of JSON for workbooks,
   workflows and actions in HTTP body.
 * Use hook for validating Content-Type
 * Changed pecan dependency on pecan>=0.8.0 due to
   hook bug in pecan v0.6.0

Implements blueprint mistral-yaml-request-body

Change-Id: I5a55aa438faeca42385e8e25770bce00da2e93a9
Merge "Use YAML text instead of JSON in HTTP body"