prompt for droplet size (1GB or 2GB RAM)

I wanted to install Discourse on a server with 2 GB of RAM, 2 GB of swap and 2 processor cores like it's recommended in the docs. So I added an option to select whether you want a server with 1GB of RAM, 1 Core and 1GB of swap or one with 2GB of RAM, 2 Core and 2GB of swap.

Thank you for open sourcing this wizard, it's super helpful!