Revamp Docker check, more metrics, less bugs, support of CoreOS

Revamp the Docker check and its configuration.

First, it fixes:

* #1031 : catch the exception
* #1027: collect 3 new metrics: containers.stopped, images.available and images.intermediate
* #1017 : is handled by the new logic 
* #1001 : allow to make Docker check work inside a container. The documentation of https://github.com/DataDog/docker-dd-agent/ will need an update. For that, we will have to mount /proc/mounts and /sys/fs/cgoup inside a directory configuration as `docker_root`.

But also:

* Now we can add optional tags in the configuration
* Clarify configuration example
* Support CoreOS

It is also a bit more flexible (if we want to support a new cgroup hierarchy) with better logging/warning.

Yeah, this parameter is useful when the Agent run inside a container. When we do that, we can mount some part of the host filesystem to a directory, which should be this `docker_root`. Then we use it as the root for our Docker daemon. By default, it is `/`, which means that we keep the normal behavior.