Extract `rbenv_prefix` to config variable.

To not duplicate to use the RBENV variables and provide custom rbenv_prefix, added ability to provide custom rbenv_prefix and now we can setup it from the `deploy.rb`.

```ruby
set :rbenv_prefix, "RBENV_ROOT=/data/apps/work RBENV_GEMSET=global ..."
```

Also in other part of deploy recipes, we can use this value:

```ruby
SSHKit.config.command_map.prefix[:custom].unshift(fetch(:rbenv_prefix))
```