Update neutron-service-ml2-plugin playbook

[itooon]

Update neutron-service-ml2-plugin playbook that add check python six library version and upgrade.

Havana neutron server need python six library version 1.4.1 or later.
However, Ubuntu 12.04 six library version is 1.1.0.

[djoreilly]

why not use the built-in idempotency that the ansible pip module provides? like:

  - name: install python-pip package
    apt: pkg=python-pip

  - name: install or upgrade python six library
    pip: name=six version=1.4.1
    notify:
      - restart neutron-server

[itooon]

In my environment, python six library was updated by other playbook to newer than 1.4.1.
So, I added version check code for supporting python six library that was upgraded by manually or other playbook.
However, this version check code has issue that is only supported by ansible 1.6 or newer.

So, I agree to use your suggested code for standard use case.