add tests for DockerDriver.get_info method

Change-Id: I9c44c77fffc0fc6e0023b61ab2d4083aa1068acb
increase performance of DockerDriver.get_info

Significantly reduce the number of requests made by one
DockerDriver.get_info call. Previously one get_info call produced N
inspect_container calls, where N is the number of currently running
containers. This solution makes only one call to the docker API method
'GET /containers/(name)/json' to inspect a container by its name:
https://docs.docker.com/reference/api/docker_remote_api/#v113.

Change-Id: Ia0ce83066e2b05fb6173d5b4a6d60f42ae57210f