stock location is never used, creates additional stock items

Change-Id: I467c0bd8f8ea8c023f850b392f28b3147cb64290
pass the product to add as a variable

Change-Id: I683de61057a82e77a5ba07478bb3a367d263f197
add order validation

validates that there is enough stock to supply all variants
fixes an issue where you could order an assembly and a part, and it
would allow you to purchase more than was in stock.
Merge pull request #5 from Dkendal/frt-ordering-assembly-and-part-oversell-fix

Frt ordering assembly and part oversell fix