Add 'memory_lxc' collector

Commit adds a new collector which collects per LXC memory utilization as well as
memory limit from '/sys/fs/cgroup/lxc'.

It produces metrics:
 * lxc.<LXC_NAME>.memory.limit_in_bytes
 * lxc.<LXC_NAME>.memory.usage_in_bytes

Signed-off-by: Zdenek Styblik <zdenek.styblik@icflix.com>