Simplify VNF Vendor Image extraction

Refactoring of the software version extraction routines to reduce
complexity.

Pass the NodeTemplates directly to the extractSoftwareVersions() method
and move the exception handling up the calling stack.
Remove the creation/usage of an intermediate Pair object.
Query for the Service VF list once only.
Use Streams rather than converting from Stream to List and back.

This change will simplify future work: either to convert sdc-tosca API
methods (that will be deprecated), or to use an alternative parser.

Change-Id: I745ced9e6095f389ee1bede37e8480dfaf65716a
Issue-ID: AAI-2281
Signed-off-by: mark.j.leonard <mark.j.leonard@gmail.com>