cluster: add Storage interface

Related to #7.
cluster: add SetStorage method

This method will define the storage. Right now, the storage is not being
used yet.

Related to #7.
cluster/test: fix mapStorage
container: use storage in CreateContainer
cluster/test: return proper error in mapStorage
cluster: add support for storage in InspectContainer

Related to #7.
cluster: use Storage in KillContainer

Related to #7.