acct-user/sshd: add new user sshd

replacing addnewgroup with addnewuser in acct-user because we are
already adding new group sshd in acct-group/sshd.

BUG=b:150325755
TESTED=emerge-lakitu acct-user/sshd and manual ssh to gce instance

Change-Id: I9af61c3db67a03f4e677b05876877a0e94695cc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/2360097
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Varsha Teratipally <teratipally@google.com>
Commit-Queue: Varsha Teratipally <teratipally@google.com>