Remove reference to distributed data store.

The compute admin guide incorrectly states that nova uses a distributed
datastore to maintain system state. I think at one point, nova used Redis,
but it currently uses a traditional database such as MySQL or PostgreSQL.

Fixes bug #912091

Change-Id: I9d6cf491b0d95ad8a88e56795fdda4fc18e26b0c

doc/src/docbkx/openstack-compute-admin/aboutcompute.xml

@@ -153,7 +153,7 @@
             response, OpenStack Compute uses asynchronous calls, with a call-back that gets
             triggered when a response is received.</para>
         
-        <para>To achieve the shared-nothing property with multiple copies of the same component, OpenStack Compute keeps all the cloud system state in a distributed data store. Updates to system state are written into this store, using atomic transactions when required. Requests for system state are read out of this store. In limited cases, the read results are cached within controllers for short periods of time (for example, the current list of system users.)</para></section>
+        <para>To achieve the shared-nothing property with multiple copies of the same component, OpenStack Compute keeps all the cloud system state in a database.</para></section>
     <section xml:id="storage-and-openstack-compute">
             <title>Block Storage and OpenStack Compute</title><para>A ‘volume’ is a detachable block storage device. You can think of it as a USB hard drive. It
             can only be attached to one instance at a time, so it does not work like a SAN. If you