log: only print timestamps when verbosity increased

While timestamps can be handy, they clutter output for normal users.
Let's print them only when verbosity (-v) is increased from default.
Currently, default is 2 (-vv) so for timestamps one should use -vvv
or -v3.

Alternatively, we could introduce a separate --timestamps option.
Personally, I find it more handy for timestamps to be tied to log level.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>