The "MAX" constants in the db api are useful to import without
needing to import the rest of the api, so they are moved to a
constants.py file in the same package. api.py then imports the
symbols from that file for backwards compatibility.
The handler and schema for placement inventories (which use the MAX
constants) are updated to point to the constants file directly as
they do not need the db api.
Change-Id: Ifab10e023bcf4389f6514b477755c4cced2d1598
431eaf6edb