Keep only 2 boxes

Production runners require 2 boxes only (latest master and ipa-4-8
branches). Nightly runners are running close to the threshold to hit
out of space issues.

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

tasks/vagrant.py

@@ -216,8 +216,8 @@ 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]
 
-        # Keep only the latest 3 boxes
-        for box in linux_boxes[:-3]:
+        # Keep only the latest 2 boxes
+        for box in linux_boxes[:-2]:
             box.delete_box()
 
     @staticmethod