improve controller: add a method to load root sub collections/resouces automatically

load_url_sub_node can load collections/resouces automatically when they
are tagged with @UrlSubNode

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
improve controller: tag the collections/resouces of root with @UrlSubNode

we can difine a /host as follow:

@UrlSubNode("host", True)
 class Host(Resource):
     def __init__(self, model, id=None):
         super(Host, self).__init__(model, id)

@UrlSubNode("host", True) means:
the URL "/host" is root.Host, and it need authentication

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
improve controller: Root loads collections/resouces automatically

Remove hard code from root.py.
We do not need to touch root.py any more when we add a new sub
collection/resouce.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
improve controller: set authentication automatically

Remove hard code from server.py.
We do not need to touch server.py any more when we add a new sub
collection/resouce.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>