[FIX] sale_coupon: reset no more eligible applied coupon to Valid state

Before this commit:
When a coupon_program's coupon was giving a reward on a SO and then the
recompute was removing that reward (eg the SO did not met the requirements
anymore) the coupon would not be reset and would be lost.

Now:
If a coupon's reward got removed for some reason, the coupon got reset to a
valid state (`new`) so it can be used again.

Step to reproduce:
  - Generate a coupon from a coupon program that needs requirements (eg $300)
  - Add that coupon to a SO that has a total over $300
  - Coupon is set as `used` and the reward is granted
  - Remove some products to set the SO total below $300
  - Recompute coupon, the reward is correctly removed but the coupon is still
    set as `used` so it won't be usable again and is basically lost.

Closes #3062

tests/test_program_rules.py

@@ -233,8 +233,10 @@ class TestProgramRules(TestSaleCouponCommon):
         order.recompute_coupon_lines()
 
         self.assertEqual(len(order.order_line.ids), 3, "The order should contains the Product A line, the Product B line and the discount line")
+        self.assertEqual(coupon.state, 'used', "The coupon should be set to Consumed as it has been used")
 
         sol1.product_uom_qty = 2
         order.recompute_coupon_lines()
 
         self.assertEqual(len(order.order_line.ids), 2, "The discount line should have been removed as we don't meet the program requirements")
+        self.assertEqual(coupon.state, 'new', "The coupon should be reset to Valid as it's reward got removed")