[Draft] Adding concurrency options to launch command

[LucindaX]

We've encountered issues while using the launch command to do with the number of requests it's making. Upon inspection of the code, we've figured out that there is no limit on the stack concurrency. As in scepter will run concurrent threads to create the stacks as much as the number of stacks are available in a batch.

Stack concurrency is determined by dependency. In case of stacks that have little to no dependencies, with large number of stacks, that could mean unlimited number of threads running concurrently, which had the adverse affect of bringing down our localstack server used for testing.

We've created a patch to be able to add an option to hard limit concurrency on the launch command. We hope this could be carried over to all relevant commands. We're still new users of sceptre, so we're unfamiliar with what other changes need to be done.

Pending :

• Adding tests