Fix self.fields on API Chassis object

All fields from objects.Chassis were being added to self.fields in
the API Chassis object. Because of this, when someone would POST,
there'd be an entry for {'id': None} in the dictionary passed to
dbapi.create_chassis(). 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 Chassis object when mapping to objects.Chassis.

Change-Id: Ie5b31455b925ae395995de7b7c638ab5e3543633
Merge "Fix self.fields on API Chassis object"