containers option does not work with Linux containers

[hrexha28]

          Hi,

I tried to use the --containers option with lxd containers but i get null in the container field for the json exorter.
Is there any way to get the linux container power number?
Thanks

Originally posted by @hrexha28 in #217 (comment)

[mathieugp]

Coincidence: I just tried the same --containers flag with the prometheus exporter on an hypervisor running libvirt-lxc only docker related cgroups are found.

It would be really really great if the scaphandre exporters could find LXC-related cgroups (through lxc/lxd or in my case libvirt-lxc), otherwise the only option that I believe would work would be to treat each container as a VM and run scaphandre inside each one.

[mathieugp]

@bpetit I can make myself available for testing new code concerning support for libvirt-lxc.

Also, as setting-up a test hypervisor running LXC containers through libvirt might be non-trivial, I can also share useful information about my machines and configurations.

[bpetit]

related to #263

Adding it to 1.1 milestone