AbstractProject.doWipeOutWorkspace() should support SCM.processWorkspaceBeforeDeletion()

Before a workspace is deleted by hudson.model.WorkspaceCleanupThread, it checks
with the SCM for the job to see if it should be allowed to delete the workspace,
by calling hudson.scm.SCM.processWorkspaceBeforeDeletion. I'm going to be adding
an override of that method in the ClearCase plugin to remove the ClearCase view
and then return true, so that we don't end up with a view still registered in
ClearCase after its view storage, etc have been deleted.

However, when "Wipe Out Workspace" is used, there is no check with the SCM
first. The workspace is just deleted. I believe that doWipeOutWorkspace in
AbstractProject should call processWorkspaceBeforeDeletion before deleting the
workspace contents, and should only proceed if processWorkspaceBeforeDeletion
returns true. This will give us the ability to remove the ClearCase view when
"Wipe Out Workspace" is used, as well as the ability for SCMs to veto the
deletion of the workspace.