Fixed time formatting in ec2/models.py (#778)

* Fixed time formatting in ec2/models.py

* Used freezegun on test that was failing due to time progression causing timestamp differences.

moto/ec2/models.py

@@ -917,7 +917,7 @@ class Ami(TaggedEC2Resource):
         self.architecture = None
         self.kernel_id = None
         self.platform = None
-        self.creation_date = datetime.utcnow().isoformat()
+        self.creation_date = utc_date_and_time()
 
         if instance:
             self.instance = instance