Allow CloudFormation stack tags to be updated

Limitations:

* does not update the tags of the resources in the stack. that can be
implemented later.
* does not support the supposed feature of clearing tags by passing an
empty value that boto3 mentions in its documentation. I could not find
anything in the request body to indicate when an empty value was passed.

moto/cloudformation/models.py

@@ -82,7 +82,7 @@ class FakeStack(BaseModel):
     def stack_outputs(self):
         return self.output_map.values()
 
-    def update(self, template, role_arn=None, parameters=None):
+    def update(self, template, role_arn=None, parameters=None, tags=None):
         self._add_stack_event("UPDATE_IN_PROGRESS", resource_status_reason="User Initiated")
         self.template = template
         self.resource_map.update(json.loads(template), parameters)
@@ -90,6 +90,10 @@ class FakeStack(BaseModel):
         self._add_stack_event("UPDATE_COMPLETE")
         self.status = "UPDATE_COMPLETE"
         self.role_arn = role_arn
+        # only overwrite tags if passed
+        if tags is not None:
+            self.tags = tags
+            # TODO: update tags in the resource map
 
     def delete(self):
         self._add_stack_event("DELETE_IN_PROGRESS",
@@ -164,9 +168,9 @@ class CloudFormationBackend(BaseBackend):
                 if stack.name == name_or_stack_id:
                     return stack
 
-    def update_stack(self, name, template, role_arn=None, parameters=None):
+    def update_stack(self, name, template, role_arn=None, parameters=None, tags=None):
         stack = self.get_stack(name)
-        stack.update(template, role_arn, parameters=parameters)
+        stack.update(template, role_arn, parameters=parameters, tags=tags)
         return stack
 
     def list_stack_resources(self, stack_name_or_id):

moto/cloudformation/responses.py

@@ -152,6 +152,14 @@ class CloudFormationResponse(BaseResponse):
             for parameter
             in self._get_list_prefix("Parameters.member")
         ])
+        # boto3 is supposed to let you clear the tags by passing an empty value, but the request body doesn't
+        # end up containing anything we can use to differentiate between passing an empty value versus not
+        # passing anything. so until that changes, moto won't be able to clear tags, only update them.
+        tags = dict((item['key'], item['value'])
+                    for item in self._get_list_prefix("Tags.member"))
+        # so that if we don't pass the parameter, we don't clear all the tags accidentally
+        if not tags:
+            tags = None
 
         stack = self.cloudformation_backend.get_stack(stack_name)
         if stack.status == 'ROLLBACK_COMPLETE':
@@ -162,7 +170,8 @@ class CloudFormationResponse(BaseResponse):
             name=stack_name,
             template=stack_body,
             role_arn=role_arn,
-            parameters=parameters
+            parameters=parameters,
+            tags=tags,
         )
         if self.request_json:
             stack_body = {