Add demote command (linkedin#91)

* Add demote command.

* More complete.

* Fixed flake8 errors for kafka/tools/assigner/actions/demote.py

The flake8 errors were part of the reason why Travis build was failing for PR linkedin#91 for linkedin/kafka-tools. The other factor was tox running into an upstream python bug [1] for which PR linkedin#94 has been raised. I confirmed that after these 2 factors were eliminated, the build passes [2].

[1] https://bugs.python.org/issue10496
[2] akashvacher@2228e65

kafka/tools/assigner/actions/demote.py

@@ -0,0 +1,56 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+from kafka.tools.assigner.actions import ActionModule
+from kafka.tools.exceptions import NotEnoughReplicasException
+
+
+class ActionDemote(ActionModule):
+    name = "demote"
+    helpstr = "Force a broker to relinquish leadership wherever it is safe to do so. This is done by reordering " \
+        "replica lists and then performing a PLE."
+
+    def __init__(self, args, cluster):
+        super(ActionDemote, self).__init__(args, cluster)
+
+    @classmethod
+    def _add_args(cls, parser):
+        parser.add_argument(
+            '-b', '--brokers', type=int, help='List of broker ids to demote', required=True, nargs='*')
+        parser.add_argument('-t', '--topics', required=False, nargs='*',
+                            help='Optional list of topics to use. If not specified, leadership of all topics lead by the brokers to demote will be changed.')
+
+    def process_cluster(self):
+        # Randomize a broker list to use for new replicas once. We'll round robin it from here
+        brokers_to_demote = set(self.args.brokers)
+        topics_to_consider = self.args.topics and set(self.args.topics) or None
+
+        for partition in self.cluster.partitions(self.args.exclude_topics):
+            if topics_to_consider and partition.topic.name not in topics_to_consider:
+                continue
+
+            if all(b.id in brokers_to_demote for b in partition.replicas):
+                msg = 'Topic {0} partition {1} only has replicas in the list of brokers to be demoted: {2}'.format(
+                    partition.topic.name,
+                    str(partition.num),
+                    ', '.join(str(b.id) for b in partition.replicas))
+                raise NotEnoughReplicasException(msg)
+
+            while partition.replicas[0].id in brokers_to_demote:
+                broker = partition.replicas[0]
+                partition.remove_replica(broker)
+                partition.add_replica(broker)