Add "/resources" endpoint

- basic support for /resources endpoint
- configurable cpu / memory allocation to agent (default to system values)

Counting reserved CPUs/Memory is marked as a TODO, since we don't yet have a way to enumerate created containers.

/cc @pb @xla @jmyounker 
`for vol := range configuredVolumes`

Are you running `golint` and `go vet`? :)