Respect work_order.can_close when confirming a sale that references it

Use workorder.can_close to check if we can confirm the sale instead of checking
if they are finished, since there are more checks to do, like checking if the
order is rejected, if it's in transport or even if it's on another branch.

Change-Id: I4db5f8108c046b3973c06d048ee360e88c2c7651