Return None for physical resource name during validation

Return None for physical resource name during validation

During template validation, the resources have not been stored in the
database. Therefore any resources that return self.physical_resource_name()
in either FnGetRef() or FnGetAtt() would cause assertions whenever either
of those functions were called during validation. Instead, return None from
physical_resource_name() in this phase, as we do for other data that is not
available yet during validation.

Fixes bug #1191931

Change-Id: I1a5b17557ae1ca2600fb930e74eb5248c82e0326