WIP: Adding endpoint for log retrieval on the minion

Adding logic for retrieving logs from docker containers.
This commit is part one on the Accessible build logs card and contains only changes necessary to query minion for desired docker container logs. 

Usage:
`curl  http://10.245.2.2:10250/containerLogs?containerID=c93b7770a0b6`

You just want to pass in the output streams as arguments to this method I think.  I.e.:

```
func getKubeletDockerContainerLogs(client DockerInterface, containerID string, out, err io.Writer) ...
```