lxccontainer: extend lxccontainer API with set_timeout

lxccontainer: extend lxccontainer API with set_timeout

lxccontainer set_timeout method allows to set LXC client
timeout for waiting monitor response.

Right now, it's implemented using the SO_RCVTIMEO client
socket option. (But it's the implementation detail that
can be changed in the future.)

This commit doesn't change behavior, because it's just
adds a new option and setter, but not changes any existing
LXC commands implementation. It's also extends internal API
function lxc_cmd with lxc_cmd_timeout.

Issue #4257

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>