Improve ETSI NS UI test

Improve ETSI NS UI test

- import a ETSI VNF that has the tosca.capabilities.nfv.VirtualLinkable
requirement;
- externalize the tosca.capabilities.nfv.VirtualLinkable requirement
and certify the imported ETSI VNF;
- on the created ETSI NS Service (already existing flow), adds two
instances of the imported ETSI VNF and a NsVirtualLink;
- create a relationship between the two ETSI VNF to the NsVirtualLink,
through the tosca.capabilities.nfv.VirtualLinkable requirement/
capability;
- checks for the new nodes and relationship in the generated ETSI NS
package.

Change-Id: I57fa3079f473c7afcb7569941f870233ce7bdcca
Issue-ID: SDC-3550
Signed-off-by: André Schmid <andre.schmid@est.tech>