fix: run only uninstall tasks, not full role on make uninstall

Tagging the whole role with uninstall caused install/configure/update/export to run first. Use include_role tasks_from uninstall.yml; add safe defaults for uninstall variables.

playbook-uninstall.yml

@@ -2,9 +2,12 @@
 - name: Uninstall Hysteria2 servers
   hosts: hysteria2_servers
   gather_facts: false
-  roles:
-    - role: hysteria2
-      tags: [uninstall]
+  tags: [uninstall]
+  tasks:
+    - name: Run Hysteria2 uninstall tasks only
+      ansible.builtin.include_role:
+        name: hysteria2
+        tasks_from: uninstall.yml
 
 - name: Rebuild global output index after uninstall
   hosts: localhost