Fix chassis-node relationship

The change 3f49758c82619b424f623f71df1ef051e50dda4c removed the chassis_id
from the Nodes.fields but it turns out that it needs to be present otherwise
the as_dict() method won't include the chassis_id field when converting
the Node object to a dictionary before saving it to the db.

Change-Id: Ice8803a2cc4955c9d9b6d786c450e112be240299
Closes-Bug: #1308320
Merge "Fix chassis-node relationship"