pool domain should always return a domain (#3707)

* pool domain should always return a domain

Refs https://github.com/spulec/moto/issues/3706

* set character encoding

* test CloudFrontDomain exists on pool domain

* describe pool domain does not return cloudfront domain

moto/cognitoidp/models.py

@@ -195,7 +195,8 @@ class CognitoIdpUserPoolDomain(BaseModel):
                 self.custom_domain_config["CertificateArn"].encode("utf-8")
             ).hexdigest()
             return "{hash}.cloudfront.net".format(hash=hash[:16])
-        return None
+        hash = hashlib.md5(self.user_pool_id.encode("utf-8")).hexdigest()
+        return "{hash}.amazoncognito.com".format(hash=hash[:16])
 
     def to_json(self, extended=True):
         distribution = self._distribution_name()

tests/test_cognitoidp/test_cognitoidp.py

@@ -137,6 +137,7 @@ def test_create_user_pool_domain():
     user_pool_id = conn.create_user_pool(PoolName=str(uuid.uuid4()))["UserPool"]["Id"]
     result = conn.create_user_pool_domain(UserPoolId=user_pool_id, Domain=domain)
     result["ResponseMetadata"]["HTTPStatusCode"].should.equal(200)
+    result["CloudFrontDomain"].should_not.be.none
 
 
 @mock_cognitoidp