Make zypper dist-upgrade opt-in on SLE/Leap (#417)

Make zypper dist-upgrade opt-in on SLE/Leap

- Create a `suse_dup` config option
- Create a new `Distribution::OpenSuseTumbleweed` object along with `upgrade_opensuse_tumbleweed()`
    * The purpose of it is to ignore the config option on Tumblweed as
      zypper `dup` is the only way to update a Tumbleweed

config.example.toml

@@ -101,6 +101,7 @@
 #emerge_sync_flags = "-q"
 #emerge_update_flags = "-uDNa --with-bdeps=y world"
 #redhat_distro_sync = false
+#suse_dup = false
 #rpm_ostree = false
 #nix_arguments = "--flake"