Enabling clustering never completes

Steps to reproduce;

1. Install a fresh Openfire.

2. Deploy the hazelcast clustering plugin.

3. Login to Openfire, navigate to the "Clustering" page. Select "Enabled", and click "Save Settings".

Expected results

• The Clustering page indicates that clustering is enabled.

Actual results

• The server stops responding. It is necessary to stop and restart Openfire, at which point the Clustering page indicates that clustering is enabled.