Fix self.fields on API Node object

All fields from objects.Node were being added to self.fields in
the API Node object. Because of this, when someone would POST, there'd
be an entry for {'id': None} in the dictionary passed to
dbapi.create_node(). We should only set fields we're exposing.

This also required fixing PATCH to not try to look at fields not set
on the API Node object when mapping to objects.Node.

A later patchset removes the direct dbapi call to create_node() in
flavor of using the objects.Node object where trying to set 'id' of None
would fail prior to this fix.

Change-Id: I92c03d9ff2a56e7999a685f9de38efec5aacbe47
Merge "Fix self.fields on API Node object"