Update cli doc: Add vitrage-purge-data.rst

In this section that only have 'vitrage-status'  on Vitrage’s command line
interface.It is easy to use the cli interface of 'vitrage-purge-data' to clear
tables, so I added this part of the document.

Change-Id: Ie74e4f1160643808a5772329c6388c4469633fd5

doc/source/cli/index.rst

@@ -9,3 +9,4 @@ interface.
    :maxdepth: 1
 
    vitrage-status
+   vitrage-purge-data

doc/source/cli/vitrage-purge-data.rst

@@ -0,0 +1,51 @@
+==================
+vitrage-purge-data
+==================
+
+---------------------------------------------
+CLI interface for Vitrage Purge Data commands
+---------------------------------------------
+
+Synopsis
+========
+
+::
+
+  vitrage-purge-data --config-file [<args>]
+
+Description
+===========
+
+:program:`vitrage-purge-data` is a tool that provides routines for clearing
+the data of a Vitrage deployment tables.
+
+Options
+=======
+
+The standard pattern for executing a :program:`vitrage-purge-data` command is::
+
+    vitrage-purge-data --config-file /etc/vitrage/vitrage.conf
+
+
+The section describe those tables that are emptied after executing the command
+
+  **Related Tables**
+
+  .. list-table::
+     :widths: 20 80
+     :header-rows: 1
+
+     * - Number
+       - Table Name
+     * - 1
+       - active_actions
+     * - 2
+       - events
+     * - 3
+       - graph_snapshots
+     * - 4
+       - changes
+     * - 5
+       - edges
+     * - 6
+       - alarms