Replace dedicated mock code with stubs (#805)

* Updated node tests involving error childnodes to use stubs
* CloudFormationNode tests now use stubs for the service clients
* CloudFormationStackNode tests now use stubs. Split up some of the tests as well.
* Added CloudFormationStackNode test to check for Error handling
* Converted LambdaNode tests to use stubs
* Converted LambdaNode to use stubs, added missing tests

Author: awschristou