Simplify OVS Plugin Tunnel Mgmt

Currently, the OVS plugin in tunnel mode requires that each compute host have a manually created file that contains all tunnel IPs in the deployment, and that all agents are restarted if that file needs to be updated.

We'll change this so that each agent "registers" its IP address in the plugin database, and that all other agents feed off this centralized database, creating tunnels as new IPs appear in the database.

Note: there's no good way to remove an IP if the the host goes away. We may need to consider having an admin utility that can purge values from the database. For the time being, items can be removed from the DB directly.