Improve overview subpage

The patch adds some information about the reason why the role
exists, what we want to achieve by the role and what are the
advantages of using it.

https://tree.taiga.io/project/tripleo-ci-board/task/490

Change-Id: I34a3603f0f8406d3e3f5fcc5b8b74825e5e1b9d1

doc/source/overview.rst

@@ -3,3 +3,42 @@ Overview
 ========
 
 .. include:: ../../README.rst
+
+
+`Tempest <https://docs.openstack.org/tempest/latest/>`_ is a testing
+framework consisting of a set of integration tests to test any deployed
+OpenStack cloud.
+
+
+os_tempest mission
+------------------
+
+To provide a re-usable ansible role which installs, configures and runs
+Tempest.
+
+
+Why?
+----
+
+The reason we have come up with this idea is because every OpenStack
+project uses playbooks and shell scripts to install, run and configure Tempest
+which are only slightly different but their purpose is the same.
+
+When every project uses its own way to use Tempest, it's really harder to
+cooperate (cross projects) together to solve any issues which may occur.
+
+That's where the re-usability steps in. By using the same role we can faster
+react to any issues which occurred in one project and may have an effect on
+another one.
+
+
+Advantages
+----------
+
+* maintenance of only one set of playbooks and scripts
+* heads-up for issues related to particular tests
+* bigger focus on development and maintenance of the one set of playbooks and
+  scripts
+* decreasing of time consumption needed to install, configure and run Tempest
+  for new OpenStack projects - no need to write their CI Tempest procedures
+  from scratch