Improve the code for creation of the dput.cf file

[Francis Ginther]

This is a follow up to https://code.launchpad.net/~ursinha/uci-engine/private-lp-mthood/+merge/218538

> > > === modified file 'branch-source-
> builder/cupstream2distro/packagemanager.py'
> >
> > > - cmd = ["dput", ppa,
> > > + cmd = ["dput", "-u", "-c", "/tmp/dput.cf", ppa,
> > > "{}_{}_source.changes".format(source,
> version_for_source_file)]
> >
> > I'm being a little over-protective, but as per: "/tmp/dput.cf" - Seems
> > like we should either make a constant for that, or (and I don't know the
> > code structure/flow, have pass this value down to the function via
> > upload_package.py
>
> I'll leave this for Francis to comment, he implemented that part and knows the
> constraints better than I do.

A constant is more appropriate and will make it easier if we need to relocate the file. And when thinking about the proper location, /srv/$service_name/etc/dput.cf is probably a better fit.