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.
This commit includes the root import feature. A `derived_from`
attribute has been included with the <type> section to make
reference to the image file to import. The image format to import
is assumed to be the same as the build type to import.
The current format supported is docker