SE-1079 Create OS and version neutral Source RPM.

Upstream src.rpm generation is not OS-independant because it uses
setup.py which checks the OS and OS version and makes decisions based
on that (upstart vs. systemd ...). That means that a Source RPM created
on Debian or on RHEL6 would not work when built on RHEL7 because the
systemd service file is missing.

Eventually it would be nice of upstream to remove these decisions from the
source building stage and put them into a binary building stage.

Till then this script will do the job.