From the Ocata upgrade guide:

Ocata requires that your deployment have created the cell and host mappings in Newton. If you have not done this, Ocata’s db sync command will fail. Small deployments will want to run nova-manage cell_v2 simple_cell_setup on Newton before upgrading. Operators must create a new database for cell0 before running cell_v2 simple_cell_setup. The simple cell setup command expects the name of the cell0 database to be <main database name>_cell0 as it will create a cell mapping for cell0 based on the main database connection, sync the cell0 database, and associate existing hosts and instances with the single cell.