LUInstanceRename: log result of name resolving

While the LU does return the final name, it's useful to log the actual
DNS resolving process (input and output) in order to help with the
diagnose of failures.

The patch also fixes the docstring of the Exec() function.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>