LU-3289 gss: gssnull security flavor

LU-3289 gss: gssnull security flavor

This change implements the gssnull security flavor for the purpose of
testing the Lustre GSS code.  It provides and uses a null GSS
mechanism so this testing doesn't have to involve any code related to
Kerberos or any other authentication method.

Signed-off-by: Andrew Korty <ajk@iu.edu>
Change-Id: Ic8378a052fd2a0f5a84877476a4a29aef7b0412a
Reviewed-on: http://review.whamcloud.com/8475
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Thomas Stibor <thomas@stibor.net>