Run taskcat tests in sequential order

[gattasrikanth]

Describe the bug
I want the ability to run my tests in sequential order in various regions.

To Reproduce
This is a feature request: With current latest version of taskcat, when I run tests in say 10 regions, all the tests trigger at the same time, meaning taskcat will start deploying cloudformation templates in all 10 regions at the same time, while this is great, it may not be useful all the times. Ex: If my solution deploys 5 S3 buckets, then running 3 tests in 10 regions will create 150 s3 buckets, so I can easily hit the resources limit. This is one example, but we have more examples of various scenarios.

It will be great if taskcat can support sequential execution of tests with optional flags so that taskcat will run one test at a time or one region at a time etc depending on how we configure, then it will be a huge help for us.

**Version (Please make sure you are running the latest version of taskcat)

• Taskcat Version 0.9.36

Note: Python Version (python3 required)