gssd: set $HOME to prevent recursion when home dirs are on kerberized NFS mount

Some krb5 routines will attempt to access files in the user's home
directory. This is problematic for gssd when the user's homedir is
on a kerberized NFS mount as it will end up deadlocked.

Fix this by setting $HOME unconditionally to "/".

Fixes this Fedora bug:

    https://bugzilla.redhat.com/show_bug.cgi?id=1052902

Reported-by: Enrico Scholz <rh-bugzilla@ensc.de>
Reported-by: nmorey <nmorey@kalray.eu>
Tested-by: Michael Young <m.a.young@durham.ac.uk>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>