vzctl set --ostemplate: require --save

Since everything vzctl set --ostemplate does is saving OSTEMPLATE to
container config, it is useless without --save.
Therefore, require --save flag to be set.

Related to https://bugzilla.openvz.org/2909

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
vzctl(8): document set --ostemplate

https://bugzilla.openvz.org/2909

Reported-by: Pavel Odintsov <pavel.odintsov@gmail.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>