ssm_testing does not allow for multiple concurrent tests

[alemartini]

Currently, the ssm_testing.py module only provides classes to create an SSM document and CFN stack.
The test logic in part of each document folder.

The ssm_testing module should provide a framework to orchestrate multiple tests of the same document with multiple parameters. The module should handle concurrency and error handling.

For example, if I wanted to test if an Automation document patches my instances correctly, I would want to test that on all supported OSes (i.e. from Windows Server 2008 R2 to the latest Windows Server release). This is currently not easy to do.