MODE-2198 - Fix clustering quickstart

I have fixed the standalone configurations and validated them against Wildfly 8.0.0.Final. 

I renamed the configuration files from standalone-modeshape-master.xml and standalone-modeshape-slave.xml to standalone-modeshape-node1.xml and standalone-modeshape-node2.xml in order to avoid confusion around the relationship between the nodes (it is not a master/slave relationship).

I replaced the jboss-as maven plugin with the wildfly one.

I updated README.md to reflect all of the above changes.

Please note that there are still some issues with clustering as discussed in https://community.jboss.org/thread/240578 but I believe this is to do with Wildfly 8 and Infinispan 6 or one of the subsystems used by Infinispan.