Fix multi-vdu workflow w/o charm

Consider the use-case where a VNF defines a vnf-configuration, and
contains two VDUs without vdu-configuration. There should only be one
charm deployed, at the VNF level.

This patch complies with the above use-case.

Change-Id: I16eadef6c4a5807d47194d4b3ee9e7c66ec67be3
Signed-off-by: Adam Israel <adam.israel@canonical.com>