requiresWorkspaceForPolling needs to be able to return false for certain builds which rely on dynamic slaves only.

To do this I have selected the value when getSlaveClientNameFormat().equals("${basename}") returns true.

Thus when a build uses a set of dynamic slaves, polling can be done on the master by 'reusing' the same workspace name for all builds, so that this workspace can also be used on the master and the p4 changes will work.

Supplied patch enables this, and has been tested in a dynamic slave only environment, where no slave is available for polling to run on.