tng-cat: Add platform field for descriptor uploads

See: https://github.com/sonata-nfv/tng-cat/issues/64

[x] build example package with 5GTANGO, OSM and ONAP descriptors
[x] extend tng-cat mime type filter functions get_content_of_type
[x] change _get_package_content_of_type to return dict: full_mime_type -> path
[x] translate mime type to tng-cat parameter and pass to request method
[x] build a unit test

curl -X POST -v -H "Content-Type: multipart/form-data" \
    -F username="tango_user1" \
    -F package="@misc/eu.5gtango.mixed-ns-package-example.0.1.tgo" \
    http://127.0.0.1:5099/api/v1/packages

Catalogue requests:

We put the fields into the query to the catalogue when uploading artifacts:

POST /foobar?platform=osm&username=user1

platform = 5gtango, osm, onap

sonata-nfv / tng-sdk-package

tng-cat: Add platform field for descriptor uploads #140