ceph_authtool.cc: update help/usage text

Added implemented but not listed commands to the help/usage text:
* -g  shortcut for --gen-key
* -a  shortcut for --add-key
* -u/--set-uid to set auid
* --gen-print-key
* --import-keyring

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
ceph-authtool.8: add missing commands to man page

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Merge pull request #474 from dalgaaf/wip-da-next-fix-help

Add missing commands to help/usage and manpage of ceph-authtool

Reviewed-by: Sage Weil <sage@inktank.com>