Detect wrong Python module structure

If, for one reason or another, a module named "ganeti.ganeti" can be
succesfully imported (e.g. through a symlink in the wrong place), error
which are diffcult to debug can occur. This piece of code tests for the
unlikely case of that happening.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>