move script specification for rpm into setup.cfg

One problem with specifying it only in the Makefile, is that it becomes
cumbersome to invoke 'python setup.py bdist_rpm' since many options are needed.
This change simplifys building RPMs and allows easier building w/o using the
Makefile. Building RPMs via the Makefile remains intact.
Merge pull request #552 from ImmobilienScout24/refactor/simplify_rpm_settings

move script specification for rpm into setup.cfg