LU-18005 gss: allow regular users to authenticate on MGS

LU-18005 gss: allow regular users to authenticate on MGS

It can be useful for regular users to be able to authenticate against
the MGS, for instance to run 'lfs check mgts'.
Just allow this type of authentication request in the code, and take
into account the MGC export when doing 'lfs flushctx', so that any
user key associated with the MGS gets flushed as well.

Add sanity-krb5 test_152 to exercise this capability.

Test-Parameters: trivial
Test-Parameters: testgroup=review-dne-selinux-ssk-part-1
Test-Parameters: testgroup=review-dne-selinux-ssk-part-2
Test-Parameters: kerberos=true testlist=sanity-krb5
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I4871b4ed61af918644e11d64ef5750a858713323
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55640
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>