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 <hu.zhijiang@zte.com.cn>