Add a step to install tox

Before use tox, we need install it.

Change-Id: I7c5c4a752beea3e6172d20e92ac4183ae4423e4b

doc/source/admin/deployment.rst

@@ -35,6 +35,13 @@ to Congress.  To generate a sample configuration file that lists all
 available options, along with descriptions, run the following commands::
 
     $ cd /path/to/congress
+
+Install tox::
+
+    $ pip install tox
+
+Generate config::
+
     $ tox -egenconfig
 
 The tox command will create the file ``etc/congress.conf.sample``, which has
@@ -114,4 +121,4 @@ The Config Datasource
 .. toctree::
    :maxdepth: 2
 
-   config-datasource
+   config-datasource