Delete unavailable py2 package

The openstack Ussuri and Victoria versions no longer support the
Centos7 and pyrhon2 environment packages. Correct the missing
problems in the latest document

Change-Id: Ie17133b86f421868cddca46c29225d51ae671594
(cherry picked from commit 19c30d86ba)
(cherry picked from commit 28f8abc29a)

doc/source/install/install-rdo.rst

@@ -177,7 +177,7 @@ Install and configure components
 
       # yum install openstack-senlin-engine.noarch \
         openstack-senlin-api.noarch openstack-senlin-common.noarch \
-        python2-senlinclient.noarch
+        python3-senlinclient.noarch
 
 2. Edit file :file:`/etc/senlin/senlin.conf` according to your system settis. The most common options to be customized include: