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
Kubernetes doesn't really have a concept of a "container instance" other than through an attemptID (read through docs/identifiers.md for an overview).  Also, clients shouldn't have to lookup an attempt ID to get access to logs - that mapping can easily be done at the node.  We probably want to focus logs on pod name + container name as the primary access pattern, but attemptID is an important secondary