Major Description
If a cluster isn't secure, then using the current Unix username isn't a strong identity. However, we still perform authorization based on that username. This makes it inconvenient to run tools like 'ksck' from a regular user account – you end up needing to 'sudo -u kudu' before doing so, and if running from a remote machine there might not even be such a user.
Given that the local username isn't a strong identity anyways (someone could always recompile kudu or use LD_PRELOAD to override it), let's provide an environment variable KUDU_USER_NAME that can perform a similar override.