Bug 18468 - When adding from a staged file order discounts are not passed into C4...

Bug 18468 - When adding from a staged file order discounts are not passed into C4::Acquisitions::populate_order_with_prices

To test:
1 - Setup a vendor with a discount
2 - Stage a file
3 - Create a basket
4 - Order from staged file
5 - Add a price but no discount
6 - Save order
7 - Note ecost is not discounted
8 - Apply patch
9 - Repeate 2-6
10 - Note ecost is discounted

Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>