JENKINS-21750 DSL support for the promoted-builds plugin

Include the possibility to generate configs to use the
"Promoted Builds Plugin". Add new DSL components and generators
for creating XML files in the place where the plugin expects it to be.
fix for promotion/downstreamParameterized/currentBuild

a promoted-builds action of downstreamParameterized
is creating the wrong type of buildSteps;
it is creating `hudson.plugins.parameterizedtrigger.TriggerBuilder`
instead of the buildStep that the web ui creates,
`hudson.plugins.parameterizedtrigger.BuildTrigger`

this subtle difference breaks the expected behavior of
passing the current build parameters via `currentBuild()`
in DSL, resulting in no parameters being passed

this fix overrides the behavior of
`AbstractStepContext#downstreamParameterized`
to behave like the PublisherContext#downstreamParameterized by
calling `DownstreamContext#createDownstreamNode(false)`

Conflicts:
	job-dsl-core/src/main/groovy/javaposse/jobdsl/dsl/helpers/promotions/PromotionContext.groovy