Take `rails_env` option into consideration

This change splits the direct connection between a `stage` option and DB
environment.

A user might have a stage file example: `config/deploy/staging_new.rb`
With this, `stage` variable is set to `staging_new` which makes no
sense.

With this update a user can:
- set the stage with `rails_env` option. This is convenient and "just
  works" if capistrano-rails gem is used.
- set the DB env explicitly with `pg_env` option. This one should not be
  necessary though.

Lastly, this change is "backwards compatible", meaning - without extra
options everything will work as until now.