Use lxc-stop -k when lxc-kill is not found

lxc-kill was removed in lxc/lxc@33ddfc2

Docker-DCO-1.1-Signed-off-by: Chia-liang Kao <clkao@clkao.org> (github: clkao)
Merge pull request #3844 from clkao/lxc-kill-deprecation

Use lxc-stop -k instead of lxc-kill