docker metrics (read metrics from cgroups for specified container)

Fixes #8842.

This is first and very simple implementation of docker metrics command. A lot of work needed to push it to production.

Roadmap:

* make find of cgroup paths more reliable (not just try to search predefined locations, but get exact value from procfs)
* specify set of metrics (from `cpuacct`, `memory`, `blkio`, ... controllers)
* store metrics in `struct` instead of `map[string]string` (typed data)
* `-f FORMAT` support
* documentation

PS. PR created for further discussion and this branch will be constantly updated.

/cc @shykes @tobegit3hub @thaJeztah
I don't see any metrics in `lxc-info` output:

    # lxc-info --name 6942bf93dd69dc47b31a7236c92c2e40afc554bd9de73efbd4a2bb2e366e2e77
    state:   RUNNING
    pid:     30634

We always used to run custom shell scripts to fetch lxc metrics from cgroups