[Digital Ocean] Offer alternative Cloud-Config for users wanting to attach secondary volumes

[ggalmazor]

It doesn't make sense to choose more expensive Droplets just to get more storage space.

For example, a $20 Droplet has 80GB SSD. Alternatively, you could get:

• A $5 Droplet with 25GB
• A $15 volume with 150GB

The $5 Droplet is probably enough to run Aggregate, and you get almost twice storage for the same price.

We could have an alternative Cloud-Config stack that configures PostgreSQL to use the secondary volume for data storage.

The Cloud-Config stack could also detect automatically this and support both scenarios (with and without secondary volume)