Update doc about how to install python-docker-py on centos
Update doc about how to install python-docker-py on centos

CentOS shipped with an older version of docker-py named docker-python before.
But currently CentOS also shipped with a new rpm named python-docker-py-1.7.2
which obsoletes docker-python and also fulfill kolla need according to its
version(kolla needs docker-py >= 1.6.0).

Change-Id: I684a6a4e6b6baa9589e9b2afd3cfcc195d2a5e21
Signed-off-by: Zhijiang Hu <b69d0d1be693fbfd2c20d35c3fa23adfe38f69bd@zte.com.cn>

rahulunair/kolla,nihilifer/kolla,mandre/kolla,stackforge/kolla,tonyli71/kolla,rahulunair/kolla,stackforge/kolla,mandre/kolla,coolsvap/kolla,mrangana/kolla,mrangana/kolla,tonyli71/kolla,dardelean/kolla-ansible,GalenMa/kolla,openstack/kolla,intel-onp/kolla,dardelean/kolla-ansible,GalenMa/kolla,mandre/kolla,intel-onp/kolla,coolsvap/kolla,nihilifer/kolla,coolsvap/kolla,dardelean/kolla-ansible,stackforge/kolla,openstack/kolla