Soft delete all duplicate rows with the same vlan except one with
the biggest value in `id` column.
Create UC on fields (vlan, deleted)
Replace Select then Insert -> Try to Insert.
Fix error in network_update method. We were able to update
vlan to duplicate.
Fix typo in description of method drop_old_duplicate_entries_from_table()
blueprint db-enforce-unique-keys
Change-Id: Ia1b58ab5a5e369a1939de0b5c2f89aee6fc2c40b