LU-16758 krb: use Kerberos machine principal in client

LU-16758 krb: use Kerberos machine principal in client

In addition to having Lustre client rely on the
lustre_root/<hostname>@REALM principal to authenticate, support the
more standard Kerberos machine principal host/<hostname>@REALM.
That avoids the need for additional keytab entries, and brings Lustre
in line with other services such as OpenSSH and NFS.

Lustre-change: https://review.whamcloud.com/50709
Lustre-commit: 74890266a39297c1c3a41263a7bfd86e0d8e426a

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Id50cef1a3a94248b958ce9ea42b5ae356f29cbf1
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/50911
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>