Build only missing Python packages

Building RPMs for all required Python packages usually
takes a lot of time. That's better to skip building Python
packages that are available as RPMs.

Python package name are converted to RPM (e.g., django to
Django and webob to python-webob) and version is checked
appropriately.

Do not force using already installed packages. It doesn't
speeds up the build process anymore, but leads to bugs.

Implements: blueprint build-only-missing
Fixes: bug #1186450
Fixes: bug #1186450

Change-Id: Id59d954eb8057746051723deee299afc2f10f01f