docker: Add docker_systemd_reload functionality

Change-Id: I8cca0cc46212ce92c474ebf67fd0835375d5d541

releasenotes/notes/docker-systemd-restart-c0b9534602460df7.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    Adds ``docker_systemd_reload`` variable (defaults to ``false``),
+    that changes restart task behaviour to reload docker instead
+    when set to ``true``.

roles/docker/defaults/main.yml

@@ -51,3 +51,6 @@ docker_registry_ca:
 # Whether to configure Docker and containerd for Zun.
 docker_configure_for_zun: false
 containerd_configure_for_zun: "{{ docker_configure_for_zun | bool }}"
+
+# Whether to restart docker systemd unit or reload
+docker_systemd_reload: false

roles/docker/handlers/main.yml

@@ -10,7 +10,7 @@
 - name: Restart docker
   systemd:
     name: docker
-    state: restarted
+    state: "{{ 'reloaded' if docker_systemd_reload | bool else 'restarted' }}"
     masked: no
   become: True