Keep only 3 boxes in the runners

Current boxes are larger than they used to be and small runners
can't handle much more than 4 boxes.

Signed-off-by: Armando Neto <abiagion@redhat.com>

tasks/vagrant.py

@@ -215,8 +215,10 @@ def clean_last_time_used(box):
 
         # Do not delete Windows boxes
         linux_boxes = [x for x in all_boxes if 'windows' not in x.name]
-        if len(linux_boxes) > 4:
-            linux_boxes[0].delete_box()
+
+        # Keep only the latest 3 boxes
+        for box in linux_boxes[:-3]:
+            box.delete_box()
 
     @staticmethod
     def installed_boxes():