Adjust REST API of descriptorgen

[stefanbschneider]

Updates to facilitate integration with SDK portal: https://github.com/sonata-nfv/tng-portal/wiki/UPD-workflow-mockups

• [x] Allow passing image names and types of individual VNFs (not just number of VNFs)
• [x] Option to generate an empty projects without any descriptors

[stefanbschneider]

Added custom image names and types in the CLI, but skipping it for now for the REST API. The problem is to pass multiple image names from the REST API to the CLI module. Apprently, argparse can easily parse multiple arguments from CLI, but not when they are passed directly. See https://stackoverflow.com/questions/54552574/argparse-use-parser-parse-args-with-nargs-and-args-rather-than-command-line-i

[stefanbschneider]

Actually, the problem wasn't in argparse, but in how the REST args were parsed and handled by Flask Restplus (I used some kind of list append parser), which is now fixed with PR #67