Added ability to set custom json

Sometimes when you deploy application you want to override default variables in Chef, for example I want to deploy specific tag. So now I can just update deploy file with next lines:
```ruby
if ENV['TAG']
  require 'JSON'
  custom_json = {
      :deploy => {
          'Application Short Name' => {
              :scm => {
                  :revision => ENV['TAG']
              }
          }
      }
  }
  set :opsworks_custom_json, JSON.dump(custom_json)
end
```