Add missing package name to import statement in OutputStreamConfig.py

The current syntax works in python2, but fails in the master--py3 nightlies.

In order to have something which works in python2 and python3, then the OutputStreamAthenaPool.OutputStreamAthenaPool module had to be renamed so that it did not have the same name as the package.
Having a module with the same name as the package name causes problems with syntax like:

from OutputStreamAthenaPool.OutputStreamAthenaPoolConf import ...

in python 2.7, so the module has been renamed to CreateOutputStreams for now and all references in the code have been adapted.