Created by: ceresek
Adds basic CORS support to the server generated with the python-flask generator (and the derived python-aiohttp and python-blueplanet). Enabled with featureCORS option (same as similar option in other generators). Disabled by default.
Mentioning TC members for Python as instructed in the PR checklist: @taxpon @frol @mbohlool @cbornet @kenjones-cisco @tomplus @Jyhess @arun-nalla @spacether.