title: Jenkins workflow job: Use parameter as branch specifier
body: <p>I want to migrate our old free style dev builds, in which we use the branch name as a build parameter, to workflow builds.
  This works fine so far, the only thing we are really missing is the ability to use the parameter, e.g.  "branch_name", as the branch specifier for the <em>Workflow script from SCM</em> section.
  On a free style build this works fine.
  Any ideas how this could be achieved? We don't want a dev to change the configuration all the time before starting a build.</p>