Fix compute_node_update() compatibility with older clients

A recent change changed the format of the data passed to conductor's
compute_node_update() method. The 'stats' element of 'values' was
changed from a dict to a JSON-encoded string. When older clients
call this method with a dict, the newer conductor puts the actual
dict in the SQL instead of a string, which breaks.

Change-Id: Ida1ad76752b1944f33c1aa9436ce36790705d6a5
Closes-bug: 1280034
Merge "Fix compute_node_update() compatibility with older clients"