Add support for SUSE build on build.opensuse.org

This adds a SUSE-specific spec file, and a suse-tarball target to the Makefile. Package maintainers (e.g.: me) would be expected to run `make suse-tarball` from a source checkout, then upload that tarball and calamari-clients.suse.spec to build.opensuse.org. See, for example, the package I have now at https://build.opensuse.org/package/show/home:tserong:calamari/calamari-clients