OAK-1013: Node.addNode(name) different behavior from JR if NodeType resolves to an abstract

Only allow the default primary type to be abstract.
Unify default type handling in TreeUtil.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1574983 13f79535-47bb-0310-9956-ffa450edef68