Verify that resources have xml:id

Add validation check for WADL files to ensure they contain an xml:id at
the parent resources. This is a new requirement that has to do with the
generation of PDFs from WADL files – without this, the paths are not
unique across WADL files.

Change-Id: Id43470e0930a3a447f1e7ab63e956541984bce0e
Closes-Bug: #1275007