The --toolkit-stack-name option can be used to specify the name for
the toolkit stack. However, since the the toolkit stack outputs
had "Export"s, which must be unique within an environment, it was
impossible to deploy multiple toolkit stacks.

This change removes the "Export"s as they are actually not used or needed
and also adds an integration test to verify that multiple toolkit stacks
can be deployed into the same environment.

`toolkitStackName` can also be specified in `cdk.json` or `~/.cdk.json`.
Updated the toolkit documentation topic to describe this.

Fixes #1416