1. --path is meant to be passed by lxc-create, but should not be passed
 in by users.  Don't advertise it in --help.
 2. --clean syntax ends up not making much sense.  Get rid of it, and
 add '--flush-cache' option instead.
Signed-off-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>