Check if entry exists in cluster_nodes before adding it

When the clustering is enabled, a new entry is added to the cluster_nodes table at startup and removed at shutdown.

But if Nuxeo is abnormally stopped, the entry is not removed from the cluster_nodes table and at the next startup, two entries with the same node id will be in the table.

Before adding an entry, we should check if an entry with the same node id exists and remove it if needed.