Allow either RunExecution (flyweight task) or AbstractBuild (with workspace)

See notes for commit 444097d65ecb97c92005b3acd63db8e03bdeb6a3.
Makes job implementation be configurable. One option allow to use a workspace.
Use maven-release-plugin 2.3.2
[maven-release-plugin] prepare release build-flow-plugin-0.12
[maven-release-plugin] prepare for next development iteration
add the possibility to read dsl from a file

This allows to store the DSL under source control with all the benefits
that this can bring. This feature is only available when the "Flow run
needs a workspace" option is enabled.

Signed-off-by: Robin Jarry <robin.jarry@6wind.com>