Remove assumption of docker runtime

[timstclair]

With the addition of rkt (#858), we should no longer make the assumption that docker is the container runtime. We should try to isolate any API fields, configuration values, and dependencies into runtime-specific sub-structures / modules.

Examples from the API:

AllDockerContainers(query *info.ContainerInfoRequest) (map[string]info.ContainerInfo, error)
DockerContainer(dockerName string, query *info.ContainerInfoRequest) (info.ContainerInfo, error)
DockerInfo() (info.DockerStatus, error)
DockerImages() ([]info.DockerImage, error)

[bluecmd]

As a continuation of this: I would like to use cAdvisor to simply scrape cgroup information from systemd, not use any Docker/rkt or other container runtime. I would appreciate if there was a "-cgroup_only" mode or something, as there is "-docker_only".
Thanks!

[timstclair]

@bluecmd see my suggestion on #1201

[timstclair]

Filed #1302 to track flag issue separately.