Rework zerotier role to prevent installing zerotier just for it to do nothing at all

This also neatly avoids things like rpm-ostree-based distros where i install zerotier in a container

roles/zerotier/tasks/main.yml

@@ -1,29 +1,36 @@
 #!/usr/bin/env ansible-playbook
 # vim:ft=ansible:
 ---
-- name: configure zerotier for apt
+- name: check for zerotier
+  stat: path=/usr/bin/zerotier-cli
+  register: zerotier_cli_path
+- name: install zerotier if we're joining networks
   block:
-    - name: ensure zerotier repo key
-      ansible.builtin.apt_key: url="{{ zerotier_repo_deb_key }}"
-    - name: ensure zerotier repo
-      ansible.builtin.apt_repository: repo="{{ zerotier_repo_deb }}"
-    - name: update apt cache
-      ansible.builtin.apt: update_cache=yes cache_valid_time=86400
-    - name: ensure packages
-      ansible.builtin.apt: name=zerotier-one
-  when: ansible_pkg_mgr == "apt"
-- name: template unit file
-  ansible.builtin.template: src=zerotier-one.service dest=/etc/systemd/system/zerotier-one.service mode=0644
-  notify: restart zerotier
-- name: join networks
-  ansible.builtin.command:
-    argv: [ zerotier-cli, join, "{{ item }}" ]
-  with_items: "{{ zerotier_networks_join }}"
-  changed_when: no
+    - name: configure zerotier for apt
+      block:
+        - name: ensure zerotier repo key
+          ansible.builtin.apt_key: url="{{ zerotier_repo_deb_key }}"
+        - name: ensure zerotier repo
+          ansible.builtin.apt_repository: repo="{{ zerotier_repo_deb }}"
+        - name: update apt cache
+          ansible.builtin.apt: update_cache=yes cache_valid_time=86400
+        - name: ensure packages
+          ansible.builtin.apt: name=zerotier-one
+      when: ansible_pkg_mgr == "apt"
+    - name: template unit file
+      ansible.builtin.template: src=zerotier-one.service dest=/etc/systemd/system/zerotier-one.service mode=0644
+      notify: restart zerotier
+    - name: join networks
+      ansible.builtin.command:
+        argv: [ zerotier-cli, join, "{{ item }}" ]
+      with_items: "{{ zerotier_networks_join }}"
+      changed_when: no
+  when: zerotier_networks_join is defined and zerotier_networks_join
 - name: leave networks
   ansible.builtin.command:
     argv: [ zerotier-cli, leave, "{{ item }}" ]
   register: zerotierleave
   with_items: "{{ zerotier_networks_leave }}"
+  when: zerotier_cli_path.stat.exists and zerotier_networks_leave is defined
   changed_when: no
   failed_when: "'0 leave connection failed' in zerotierleave.stdout"