Flow project support

The approach is pretty straigh forward, it consists of one new
module handling the project-type "flow". It supports the only field
needed by this kind of project an XML block containing the flow
description in a custom DSL.

Change-Id: I5195391609b3058ae9e24a59024e0cc7a69b16a0
Implements: blueprint jenkins-job-builder-flow
Merge "Flow project support"