Add <distributable/> to shared-session-config schema

Currently, the shared-session-config does not indicate whether a distributable or non-distributable session manager should be used. Consequently, we assign the session manager implementation based on the availability of the module providing the distributed implementation. Since the default infinispan web session configuration attempts to passivate sessions on shutdown, users will see NotSerializableExceptions on shutdown if they use non-serializable session attributes - even if their web applications do not define themselves as being <distributable/>.
To clarify this ambiguity, we should add a <distributable/> element (a la web.xml) to the shared-session-config schema to determine which session manager implementation to use.