sequence gap on delivery orders (chained location)

Hello,

Tested on 7.0 : addons rev 9315 - server rev 5034

Description
- - - - - - - - - - - - - - - - - - -
When we configure chained locations, it generates a sequence gap on delivery orders :
OUT/00002 --> OUT/00004 --> OUT/00006, etc...

Steps to reproduce
- - - - - - - - - - - - - - - - - - -
On runbot.openerp.com,
- Create a new db with demo data
- Install module:
  - Sales management
  - Warehouse
- Put admin user in extended view (Technical Features")
- In warehouse configuration select:
  - Manage multiple locations and warehouses
- Go to Warehouse -> Configuration -> Location, select Output location
 - Edit chained location customer: Set "Chaining type" to "Manual operation"
- Create 2 sale orders with field "Create Invoice" on Delivery Order.
- Confirm Sale Orders, it generates two delivery orders with a sequence gap (OUT/00002 and OUT/00004)

Romain