Bug 2038 fixed

The method _format_additional_params of the instance_topics.py was
changed in order to verify the target object (VNF|NS|VDU|KDU) where
we need to populate the params with the additional ones given by the
user

Change-Id: I599bb6320054e15917e11d81d6971b33f13f20fd
Signed-off-by: Pedro Escaleira <escaleira@av.it.pt>