Allow locust benchmarks to use zeromq

The locust benchmarks currently produce this warning:

    UserWarning: WARNING: Using pure Python socket RPC implementation
    instead of zmq. If running in distributed mode, this could cause a
    performance decrease. We recommend you to install the pyzmq python
    package when running in distributed mode.

http://docs.locust.io/en/latest/installation.html#installing-zeromq

Change-Id: Ib5a0459f9f5fd74cfb2cf053417753edefc9c2b0

tests/test-benchmark-keystone-upgrade.yml

@@ -19,7 +19,10 @@
   tasks:
     - name: Install locust
       pip:
-        name: locustio
+        name: "{{ item }}"
+      with_items:
+      - locustio
+      - pyzmq
     - name: Drop benchmarking config files
       template:
         src: "{{ item.src }}"