Branch HEAD

Branch HEAD
b=19413
i=eric.mei
i=yong.fan

Fix setup_kerberos.sh to:
1) create principal for MGS and add it to the Kerberos database
2) create and install the keytab file krb5.keytab on the MGS node

Fix sanity-gss.sh test 150 to:
1) support running on multiple client nodes
2) unmount one mount point ($MOUNT) on each client node
3) not to leave $MOUNT unmounted at the end of the test

Fix t-f zconf_mount_clients() to return the correct value.