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`

@smarterclayton @csrwng so can we use a go channel here to stream the buffer up to the http handler and out?  or is "buf" already going to have allocated memory for the entire log anyway, even before Bytes() is called?