Dynamic Scaffolding: Make hard limit of 6 properties in index.gsp configurable

Currently the template of the `index.gsp` contains a hard limit of maximum `6` domain properties. This is not very flexible since I as developer cannot change the behaviour.

This hard limit would furthermore force me to `install-templates` which not what I want only to tweak this small configuration. This pull request contains a simple improvement and give the developer the freedom to override the hard limit of `6` by adding this statement `grails.plugin.scaffolding.properties.max = 20` to the `Config.groovy`.