[Resolve Sceptre#915] Support ChangeSetType UPDATE

This adds logic supporting the case of creating change sets for a stack
that does not already exist, i.e. specifying ChangeSetType of CREATE.
https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/cloudformation/client/create_change_set.html

sceptre/plan/actions.py

@@ -440,6 +440,17 @@ def create_change_set(self, change_set_name):
         :param change_set_name: The name of the Change Set.
         :type change_set_name: str
         """
+        try:
+            existing_status = self._get_status()
+        except StackDoesNotExistError:
+            existing_status = "PENDING"
+
+        self.logger.info(
+            "%s - Stack is in the %s state", self.stack.name, existing_status
+        )
+
+        change_set_type = "CREATE" if existing_status == "PENDING" else "UPDATE"
+
         create_change_set_kwargs = {
             "StackName": self.stack.external_name,
             "Parameters": self._format_parameters(self.stack.parameters),
@@ -449,6 +460,7 @@ def create_change_set(self, change_set_name):
                 "CAPABILITY_AUTO_EXPAND",
             ],
             "ChangeSetName": change_set_name,
+            "ChangeSetType": change_set_type,
             "NotificationARNs": self.stack.notifications,
             "Tags": [
                 {"Key": str(k), "Value": str(v)} for k, v in self.stack.tags.items()