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>

doc/quickstart.rst

@@ -136,8 +136,16 @@ For Ubuntu 14.04 which uses upstart instead of systemd, run the following:
 
 
 On the target hosts you also need an updated version of the Docker python
-libraries. CentOS currently ships with an older version. Using pip it is
-easy to install a newer version:
+libraries:
+
+.. NOTE:: The old docker-python is obsoleted by python-docker-py.
+
+::
+
+    yum install -y python-docker-py
+
+
+Or using pip to install a latest version:
 
 ::