Option to reset DB, plus set unicorn rails env

This adds an attribute that allows db:drop then db:setup (for creating a new environment). Also adds an attribute that sets the rails env for unicorn.

The default state of each attribute means no difference in behaviour from existing rails-lastmile.

I'm using this to set up local dev envs with mysql as well as production ec2 instances with rds (using https://github.com/nrako/librarian-ec2).