[rpmbuild] add --copr arg to build/dump-configs against copr+chroot build defs
* note that there some problems yet to be solved:
- /api_3/project-chroot endpoint does not return copr own repos (base+devel) and it also does not include copr additional repos
- priority handling is left to be done
- root mock config var is 'None' for --copr switch
* also:
- frontend_url = https://copr.fedoinfracloud.org should be used currently due to production redirect setup problem