Have HBase shell warn users when cell visibility and simple authentication are in use together

When simple authentication and cell visibility are enabled, it's pretty easy for users (particularly new ones) to accidentally log in as the same user that is running HBase. That can lead to it looking like cell visibility isn't working when it really is.

This is probably an issue mainly for local installations on development machines, but it should show a warning saying something like "HBase is running as {principle} and you logged in as {principle}. Cell visibility is enabled, but will return all rows because you are acting as the superuser."