vzctl create: default layout is ploop

If layout is not set (in command line or global config), and ploop
is supported, use ploop by default.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Released vzctl 4.7

Changes: see http://openvz.org/Download/vzctl/4.7/changes

Signed-off-by: Kir Kolyshkin <kir@openvz.org>