Switch to python3-gunicorn and Fedora 27
[a44a16f] switched to Python 3 on Fedora. This made
gunicorn not finding waiverdb.wsgi .
Switching to python3-gunicorn solves this.
Following futureproofing from [42754a4], change the base
image to be fedora:27 .
Signed-off-by: Hunor Csomortáni <csomh@redhat.com>