Bring back custom migrate environment variable

With Capistrano 2.X it was possible to define custom 
environment variables for the migration. This commit
brings back this feature.
https://github.com/capistrano/capistrano/blob/legacy-v2/lib/capistrano/recipes/deploy.rb#L438
There is some special DSL to apply ENV vars, please read SSHKit docs and especially `with` method