Refine the overview.rst for sahara
Refine the overview.rst for sahara

The change document is
https://github.com/openstack/sahara/blob/master/doc/source/overview.rst
In the section of introduction,the description of key feature's
formate is not unified.The leading is bigger than others, and the
third point is overstriking.Why the line is overstriking?

Change-Id: I006ad5a887597a02a06b52784bd82b5bb863dbb2

egafford/sahara,tellesnobrega/sahara,openstack/sahara,tellesnobrega/sahara,openstack/sahara,egafford/sahara