This fixes#4141, and pave ways for future changes around changeset.
We had subclassed FakeChangeSet from FakeStack, not from BaseModel. This made
us easier to send the response for describe_change_set calls, but when we are
handling the details of change set, the old approach won't work at all.
For example, when we were creating a changeset, we were actually creating a
stack without registering it (self.stacks), and future update onto this stack
is not really possible.
Signed-off-by: Kai Xia <kaix+github@fastmail.com>
9af1a96174