Adding endpoint for log retrieval on the minion

Adding logic for retrieving logs from docker containers.
Usage:
To returns snapshot of the logs:
`curl http://10.245.2.2:10250/containerLogs?containerID=c93b7770a0b6`  

 To tail 15 lines of logs:
`curl http://10.245.2.2:10250/containerLogs?containerID=c93b7770a0b6&tail=15`

To stream logs:
`curl http://10.245.2.2:10250/containerLogs?containerID=c93b7770a0b6&follow=1`

Related: 
https://github.com/GoogleCloudPlatform/kubernetes/issues/1235#issuecomment-55188947
At this moment, UUID is used as attemptID to have time uniqueness. In the future, we are still finalizing attemptID. You can monitor https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/api/v1beta3/types.go for more discussion. 

For now, I think you cloud have "podID+containerName" and "podID+attemptID(a.k.a. UUID)+containerName". For podID+containerName case, you can retrieve the latest attempt's log from docker. @smarterclayton how do you think?