Support appending/prepending to a test stack name

[jarosser06]

It would be useful if you can add additional information to a CloudFormation Stack name especially when multiple tests are running.

jetstream --test -m my_package --test-name ${CIRCLE_BRANCH}

Which would result in a test name of "jetstream-test-1475497601-my_feature"

@martinb3 @horacio3 Thoughts?

[twistedgrim]

I was just trying to find a stack among many stacks because of this issue.

While the epoch timestamp is a nice unique identifier I think we might be able to substitute a variable given from --test-name instead of "timestamp" . I would start to be worried about length of the cloudformation stack name. I like the idea to prepend, but I think append would be easier if you have quite a few different stacks tested among the set.

https://github.com/rackerlabs/jetstream/blob/master/jetstream/testing.py#L43