Add docker-logins config to kubernetes-worker

cluster/juju/layers/kubernetes-worker/config.yaml

@@ -49,3 +49,12 @@ options:
         runtime-config=batch/v2alpha1=true profiling=true
       will result in kube-apiserver being run with the following options:
         --runtime-config=batch/v2alpha1=true --profiling=true
+  docker-logins:
+    type: string
+    default: "[]"
+    description: |
+      Docker login credentials. Setting this config allows Kubelet to pull images from
+      registries where auth is required.
+
+      The value for this config must be a JSON array of credential objects, like this:
+        [{"server": "my.registry", "username": "myUser", "password": "myPass"}]

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

@@ -14,6 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import json
 import os
 import random
 import shutil
@@ -435,6 +436,32 @@ def extra_args_changed():
     set_state('kubernetes-worker.restart-needed')
 
 
+@when('config.changed.docker-logins')
+def docker_logins_changed():
+    config = hookenv.config()
+    previous_logins = config.previous('docker-logins')
+    logins = config['docker-logins']
+    logins = json.loads(logins)
+
+    if previous_logins:
+        previous_logins = json.loads(previous_logins)
+        next_servers = {login['server'] for login in logins}
+        previous_servers = {login['server'] for login in previous_logins}
+        servers_to_logout = previous_servers - next_servers
+        for server in servers_to_logout:
+            cmd = ['docker', 'logout', server]
+            subprocess.check_call(cmd)
+
+    for login in logins:
+        server = login['server']
+        username = login['username']
+        password = login['password']
+        cmd = ['docker', 'login', server, '-u', username, '-p', password]
+        subprocess.check_call(cmd)
+
+    set_state('kubernetes-worker.restart-needed')
+
+
 def arch():
     '''Return the package architecture as a string. Raise an exception if the
     architecture is not supported by kubernetes.'''