split docker-logins logic into 2 handlers

Previous commit relied on non-obvious reactive behavior, with the
'docker.restart' flag being exploited to re-execute docker_logins_changed
in a single hook execution.

Split this into 2 handlers:
- when docker-logins config changes, set one flag to handle the login
and another to request a daemon restart (if needed).
- run docker login after any potential daemon restarts have occurred.

cluster/juju/layers/kubernetes-worker/reactive/kubernetes_worker.py

@@ -451,16 +451,27 @@ def extra_args_changed():
 
 
 @when('config.changed.docker-logins')
-@when_not('docker.restart')
 def docker_logins_changed():
-    """Login to a docker registry with configured credentials."""
+    """Set a flag to handle new docker login options.
+
+    If docker daemon options have also changed, set a flag to ensure the
+    daemon is restarted prior to running docker login.
+    """
     config = hookenv.config()
 
     if data_changed('docker-opts', config['docker-opts']):
-        hookenv.log('Skipping docker login until daemon is restarted.')
+        hookenv.log('Found new docker daemon options. Requesting a restart.')
         # State will be removed by layer-docker after restart
         set_state('docker.restart')
-        return
+
+    set_state('kubernetes-worker.docker-login')
+
+
+@when('kubernetes-worker.docker-login')
+@when_not('docker.restart')
+def run_docker_login():
+    """Login to a docker registry with configured credentials."""
+    config = hookenv.config()
 
     previous_logins = config.previous('docker-logins')
     logins = config['docker-logins']
@@ -482,6 +493,7 @@ def docker_logins_changed():
         cmd = ['docker', 'login', server, '-u', username, '-p', password]
         subprocess.check_call(cmd)
 
+    remove_state('kubernetes-worker.docker-login')
     set_state('kubernetes-worker.restart-needed')