LU-4371 gss: Generalize testing of GSSAPI code

Rename sanity-gss.sh to sanity-krb5.sh and make sanity-gss.sh a
mechanism-agnostic test suite for just the GSSAPI code itself using
the gssnull security flavor, which in turn uses the null GSSAPI
mechanism.

Test-Parameters: testlist=sanity-gss
Signed-off-by: Andrew Korty <ajk@iu.edu>
Change-Id: I149ce66c4b044e20b9e8e1b93058ec4f62aba881
Reviewed-on: http://review.whamcloud.com/8525
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>