Fix not correct AWS::StackId value

Add call of method _set_param_stackid() to method update_task() in class
parser.Stack. This ensures that an updated stack still has the right
AWS::StackId param.
Add a unit test for this case.

Change-Id: I0d28346631546577a85735292a24c2512476d4b4
Closes-Bug: #1240888
Merge "Fix not correct AWS::StackId value"