Adds Zaqar consumer-producer scenario

This scenario for Zaqar benchmarks a serial consumer-producer
use case: creates a queue, sends messages to it, retrieves
messages from it and deletes the queue.

Some changes have been added to the fakes to test this
scenario properly.

Change-Id: I49d5956bbf571840829b3e9b1411a907748cf1ed

rally-jobs/rally-zaqar.yaml

@@ -10,3 +10,17 @@
       sla:
         failure_rate:
           max: 0
+
+  ZaqarBasic.producer_consumer:
+    -
+      args:
+        name_length: 10
+        min_msg_count: 50
+        max_msg_count: 200
+      runner:
+        type: "constant"
+        times: 100
+        concurrency: 10
+      sla:
+        failure_rate:
+          max: 0

tests/unit/fakes.py

@@ -248,10 +248,13 @@ class FakeQueue(FakeResource):
         self.queue_name = name
         self.messages = FakeMessagesManager(name)
 
-    def post_message(self, messages):
+    def post(self, messages):
         for msg in messages:
             self.messages.create(**msg)
 
+    def messages(self):
+        return self.messages.list()
+
 
 class FakeDbInstance(FakeResource):
     pass