provision/docker: passing cluster storage and stop using docker client directly
provision/docker: creating helper functions for clear storage and create containers on storage

Also passing storage to cluster when creating a new instance.
provision/docker/docker_test.go: creating fake containers also in storage
provision/docker/docker: removing replicate image test without registry url

This doesn't make sense because the user must add the registry url on
the image name, or he/she won't be able to push the image to the
registry, and so not replicate it.
provision/docker/docker.go: stops storing registry, it's not needed anymore
provision/docker: creating fake containers when inserting them
provision/docker/docker_test: finished merging. Thanks git
provision/docker/suite_test.go: created helper functions when using redis storage for docker cluster