lxc-monitord: remove hard code execvp path of lxc-monitord

Sometimes, the path of lxc tools is not '/usr/bin', but
'/usr/local/bin' or other. Then execvp lxc-monitord will fail
in lxc_monitord_spawn.

Signed-off-by: Rui Xiang <rui.xiang@huawei.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>