[TEST] Added support for multi-vnf macro instantiation

[TEST] Added support for multi-vnf macro instantiation

Decoupled service YAML template into a model YAML template and a (SO) service YAML template.
Model YAML template will be used during the onboarding steps and service YAML template will be used to generate payload when sending instantiation request to SO.
Service YAML template reference model name to use for its VNF/VF-Modules using "model_name" field.
This provide more flexibility to design the testcase and enables to setup more complex testcases like instantiating several VNF/VF-MOdules using the same SDC model infos.
This patch aims to provide backward compatibility for existing testcases based on YAML template.

Issue-ID: TEST-358
Signed-off-by: Julien Fontaine <julien.fontaine@bell.ca>
Change-Id: I69d370eff4d383d5af135206476c65e4a56e4ee5