NIFI-5973 More comments and better defaults for the shell provider.
NIFI-5973 Fixed bug where user was being retrieved by identifier when identity was provided.
NIFI-5973 Fixed a formatting string in the OS X shell commands.
Updated testing conditions to run IT in OS X environment.
Changed unit test to provide identity rather than identifier.
This closes#3537.
Signed-off-by: Andy LoPresto <alopresto@apache.org>