Add unique constraints to Cell.

In documentation: "name: Name of the current cell. This must be
unique for each cell"
Added unique constraint 'uniq_cell_name0deleted' ('name','deleted')
to Cell model and migrate sripts.
Added new exception `CellsExists`. Updated cell_create() in db.api.
Tests updated respectively.

blueprint db-enforce-unique-keys

Change-Id: Ib76e167089cb4756e4438613b49013ebf0874afe
Merge "Add unique constraints to Cell."