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

src/config.rs

@@ -275,6 +275,7 @@ pub struct Linux {
     apt_arguments: Option<String>,
     enable_tlmgr: Option<bool>,
     redhat_distro_sync: Option<bool>,
+    suse_dup: Option<bool>,
     rpm_ostree: Option<bool>,
     emerge_sync_flags: Option<String>,
     emerge_update_flags: Option<String>,
@@ -996,6 +997,15 @@ impl Config {
             .unwrap_or(false)
     }
 
+    /// Use zypper dist-upgrade (same as distro-sync on RH) instead of update (default: false on SLE/Leap, ignored on Tumbleweed (dup is always ran))
+    pub fn suse_dup(&self) -> bool {
+        self.config_file
+            .linux
+            .as_ref()
+            .and_then(|linux| linux.suse_dup)
+            .unwrap_or(false)
+    }
+
     /// Use rpm-ostree in *when rpm-ostree is detected* (default: true)
     pub fn rpm_ostree(&self) -> bool {
         self.config_file