using external database causes nats to update alongside other components

[thomasmitchell]

The database vm(s) which normally update after nats are marked as serial, and so under normal deployment conditions, the nats vms would update, then the database would update, and then capi, diego, and other components would have a wave of updates. When the database instance_group is removed because of using an external database, nats updates at the same time as the components afterward that depend on it.

When an external database is used, the nats vms should be in their own serial group.