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

@@ -1,6 +1,11 @@
 #!/usr/bin/env ansible-playbook
 # vim:ft=ansible:
 ---
+- name: check for zerotier
+  stat: path=/usr/bin/zerotier-cli
+  register: zerotier_cli_path
+- name: install zerotier if we're joining networks
+  block:
     - name: configure zerotier for apt
       block:
         - name: ensure zerotier repo key
@@ -20,10 +25,12 @@
         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"