rsync's "compress" option just does not make any sense when rsync is
used to copy files locally, it only increases CPU usage and slows down
the process ;-)
This commit adds support for OCI images. Most of the docker related
code is reused for OCI classes and Docker classes have been refactored
so now they are a splecialization of the OCI classes. It is done this
way since KIWI internally only uses OCI format to operate with
containers, therefore docker images just differ from OCI images by
the way they are packaged or unpackaged.
