Integration tests should execute on an isolated CLI installation

[cruhland]

When running the integration tests, I noticed that some of them use the standard config directory (~/.dcos) because I had failures due to my configuration. For example, I had the kafka subcommand installed, and that resulted in the test for the "top level" menu output failing.

The integration tests should always use a CLI that has been installed into a temporary directory, and configured to use generated, test-specific resources. That would prevent unwanted interactions between the tests and any actual CLI installations on developer machines.