container: change CreateContainer to also return the ID of the node
container: added InspectContainer method

This first version is very dumb: at the moment, the cluster does not
store any information about containers, so it just ask all nodes about
the given containers, returning any error that is not ErrNoSuchContainer
as the result.

This isn't the ideal solution, obviously.
all: gofmt -s -w .
container: add KillContainer method to the Cluster type