Configure frontend:application default plugincluster (child node) explicitly

A CMS application (frontend:application) initialization is currently based on the first (frontend:plugincluster) child node:
this becomes the "default cluster" of the JcrConfigServiceFactory returned by the JcrApplicationFactory.

Practically this requires for the "cms" [1] application that the "cms-static" [2] plugincluster always must be its first child node, otherwise the CMS simply won't 'start' properly, resulting in a blank screen.

This however is quite cumbersome and brittle as the (bootstrap) order of the children under the "cms" node must be ensured. 

Technically however this wouldn't be needed if the frontend:application would (optionally) just have a property telling it which child node to be used as "default cluster".
There really is no (other) usage requiring the child nodes to be orderable for a frontend:application node.

This improvement will solve the above as follows:

• add a new frontend:defaultplugincluster (String) property to the frontend:application node type
• remove the orderable qualifier from the frontend:application node type (this is  a backwards compatible trivial node type change)
• update the frontend.cnd initializate item to reload on bootstrap
• add new hippo:contentpropset initialize items for the cms, console and logic frontend:application configuration items to configure their frontend:defaultplugincluster property

[1] /hippo:configuration/hippo:frontend/cms
[2] /hippo:configuration/hippo:frontend/cms/cms-static