JENKINS-22892 - DSL for initial Base ClearCase support

Here a first version to support also the ClearCase plugin. It currently it supports *Base ClearCase* (not UCM).

I would have a questions w.r.t. if there is a good practice/convention with mutual exclusive parameters in the job-dsl-plugin.

Example:
The clearcase plugin allows to specify the config spec either be directly adding the configuration to a text field or one can select a file. Both parameter are mandatory but only one at a time can be specified.

Is there a good way to make sure that exactly one is specified? Maybe a check in javaposse.jobdsl.dsl.helpers.ScmContext#baseClearCase(groovy.lang.Closure)?