Generate RPM spec file along with source distribution archive

Utility of the spec file is increased if it has python-netsnmpagent's version number
already specified -- the old way of requiring a RPM "--define" only worked when the
Git repo was available. Now one can take the generated dist/python-netsnmpagent.spec
along with the source archive and simply copy it some build host and build.
Collect RPMs built in dist/ and clean up RPM build dirs
Automatically generate %changelog section for RPM spec file

The spec file uses a different changelog style than the ChangeLog file, which would be too
verbose for this purpose. Usually package maintainers note changes to the spec file itself
here as well and often give a more high-level summary of changes of the software being
packaged. Our automatically generated changelog will not be of the same quality as we only
indicate the timestamps a version was tagged and an "Update to version x.y.z"-style message.
But until now we didn't use the ChangeLog section at all. And package maintainers can still
replace it with a more detailed, manually maintained version.