Added LinkNodes validation to CreateNodeController

src/main/java/it/inaf/oats/vospace/CreateNodeController.java

src/main/java/it/inaf/oats/vospace/CreateNodeService.java

src/main/java/it/inaf/oats/vospace/URIUtils.java

...t/java/it/inaf/oats/vospace/CreateNodeControllerTest.java

...st/java/it/inaf/oats/vospace/persistence/NodeDAOTest.java

src/test/resources/create-external-link-node.xml

src/test/resources/create-internal-link-node.xml

src/test/resources/create-link-node-notarget.xml