SEC-920: Update preauth sample to make use of internal authentication manager

http://jira.springframework.org/browse/SEC-920. Updated context file to use <custom-authentication-provider>.

samples/preauth/src/main/webapp/WEB-INF/applicationContext-security.xml

@@ -18,18 +18,13 @@
         </sec:filter-chain-map>
     </bean>
 
-    <bean id="authenticationManager" class="org.springframework.security.providers.ProviderManager">
-        <property name="providers">
-        <list>
-            <ref local="preAuthenticatedAuthenticationProvider"/>
-        </list>
-        </property>
-    </bean>
-
     <bean id="sif" class="org.springframework.security.context.HttpSessionContextIntegrationFilter"/>
-
-    <bean id="preAuthenticatedAuthenticationProvider" class="org.springframework.security.providers.preauth.PreAuthenticatedAuthenticationProvider">
-        <property name="preAuthenticatedUserDetailsService" ref="preAuthenticatedUserDetailsService"/>
+
+    <sec:authentication-manager alias="authenticationManager" />
+
+    <bean id="preAuthenticatedAuthenticationProvider" class="org.springframework.security.providers.preauth.PreAuthenticatedAuthenticationProvider">
+        <sec:custom-authentication-provider />
+        <property name="preAuthenticatedUserDetailsService" ref="preAuthenticatedUserDetailsService"/>
     </bean>
 
     <bean id="preAuthenticatedUserDetailsService"