Preserve Hazelcast network settings between plugin upgrades

Currently, the Hazelcast plugin stores the Hazelcast networking configuration in the file `hazelcast/classes/hazelcast-cache-config.xml` - which means when upgrading the plugin it's necessary to keep a backup of that file somewhere safe, update the plugin, let Openfire unpack the JAR file, shut down Openfire and re-edit the file.

It would be better if the network settings were preserved during the upgrade process.

The newer version of Hazelcast allows configuration files to be imported; I propose to copy a template of the network settings currently contained in the existing XML file to `$OPENFIRE_HOME/conf/hazelcast-cache-config.xml` if it does not already exist, and include that file from the file delivered with the plugin.