The support for delta_root allows to build a delta container
image from a given base container. Due to the refactoring of
the kiwi code base using context managers no explicit deletion
of instances happens anymore. This uncovered a weakness of
the delta root code at the level of the overlay mount. At
the time of the umount there are still active temporary
mount handlers which keeps the mountpoint busy. In order to
fix this properly also the PackageManager factory is now
a context manager and the Repository factory received a
cleanup method which is called when the PackageManager goes
out of scope. This refactoring also fixes the busy state
when building deltas
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