fix(layout): limit region scope to the layout element

The idea here is to fix the region scope to the layout container. 

The difference from #1685 is that the regions $el are "rescope" after layout has been rendered and not during initialization, so we are sure that layout.$el is defined.

The trick is to override the `getEl()` implementation to use Backbone.$ only as a fallback and to use the `this.$el.find` as it's already done in `Backbone.View`.

related to #1685
Little bit more cleanup, and passes unit tests. Need to write a failing unit test still

https://github.com/megawac/backbone.marionette/compare/525bc2d3...8eff8b5ff1