Use build_app from app.py, not from wsgi.py

For tests it's important to use `build_app` from app.py because it doesn't
use the `HTTPLoggerMiddleware`. This makes us possible to run tests without
write permissions in `/var/log/nailgun`.

The commit also removes the obsolete `paste` package from dependencies.

Change-Id: I401ef74abc95961d5687d12835752b881ef0e7ef
Closes-Bug: #1396685
Merge "Use build_app from app.py, not from wsgi.py"