Add filter_split_soa.py for soa integration-tests split

This script is to be run like this:
python filter_split_soa.py PATH_TO_INTEGRATION_TESTS_REPO \
PATH_TO_NEW_REPO soa|non-soa
Depending on the last argument you either remove everything but soa
or you keep everything but soa plugins.
As a prerequisite you need all branches to be created locally in
the original repo clone.