OpenStack API: Replaced Flask with WSGI

Flask server introduced a couple of problems, e.g., pending
sockets when an experiment was shut down.
This patch solves this and improves stability and performance
of the emulated OpenStack APIs.

Increased PIP timeout in Dockerfile to make build more
robust in CI environment.

Change-Id: I54e040c56a10075555d13fe6fb144fc117340026
Signed-off-by: peusterm <manuel.peuster@uni-paderborn.de>