In preparation to support creation of native docker containers the
way docker likes it, the kiwi schema has to be adapted. So far there
was no way to specify additional metadata information for containers.
With this commit a new section called <containerconfig> is provided.
containerconfig is a subsection of the type and only allowed for
the docker image type. This constraint is checked via a schematron
rule. In addition an xslt stylesheet moves the currently existing
container attribute, which was used to set a name for the container,
into the new containerconfig section as name attribute