Elastic IP PhysicalResourceId should always be its public IP (#841)

According to the [CloudFormation `Ref` docs][docs], the `Ref` return
value (and physical ID of the resource) for an Elastic IP is its public
IP address.

[docs]: http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-ref.html

moto/ec2/models.py

@@ -2836,7 +2836,7 @@ class ElasticAddress(object):
 
     @property
     def physical_resource_id(self):
-        return self.allocation_id if self.allocation_id else self.public_ip
+        return self.public_ip
 
     def get_cfn_attribute(self, attribute_name):
         from moto.cloudformation.exceptions import UnformattedGetAttTemplateException