Allow to start service via node attribute

@andruby I currently have to resort to a pretty ugly hack to get MailCatcher started:

https://github.com/elm-city-craftworks/practicing-ruby-cookbook/blob/61fc1ecf7fd61b5f7aeb05845b55ff9663893dc8/recipes/_mailcatcher.rb#L14-L19

This pull request adds a new attribute that -- when set to true -- will cause the service to start immediately after installation. The attribute is set to false by default, so the current behavior is not changed at all.

/cc @sandal