Lock down ansible version

We can lock down to this alpha version as long as it works once. That
is the value of building an image here. We likely won't run into any
bugs if we can deploy from this tag at least once.

Change-Id: I864290260d7fbdcd8325748788d2454d1dc1a699
Closes-Bug: #1501455

docker/kolla-ansible/Dockerfile.j2

@@ -19,7 +19,7 @@ RUN pip install -U pip wheel
 
 RUN pip --no-cache-dir install shade
 
-RUN git clone --depth 1 https://github.com/ansible/ansible.git \
+RUN git clone --depth 1 -b v2.0.0-0.2.alpha2 https://github.com/ansible/ansible.git \
     && cd ansible \
     && git submodule update --init --recursive \
     && pip --no-cache-dir install .