pushimage: prompt the user when uploading to the release bucket

It's rare that people want to run this and upload to the actual release
bucket.  It's much more common they typod the argument list.  Prompt the
user to verify this is truly what they want to protect the system.

BUG=None
TEST=`pushimage --board=x86-alex gs://chromeos-image-archive/asdfasdfasdf` prompts nicely
TEST=`pushimage --board=x86-alex -n gs://chromeos-image-archive/asdfasdfasdf` does not prompt
TEST=`pushimage --board=x86-alex -M gs://chromeos-image-archive/asdfasdfasdf` does not prompt
TEST=`pushimage --board=x86-alex --test-sign-mp gs://chromeos-image-archive/asdfasdfasdf` does not prompt

Change-Id: If8ce8821acb613a9d0d42b7765c822aac7376864
Reviewed-on: https://chromium-review.googlesource.com/186160
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/pushimage_unittest.py b/scripts/pushimage_unittest.py
index 3cd9849..f393346 100755
--- a/scripts/pushimage_unittest.py
+++ b/scripts/pushimage_unittest.py
@@ -239,7 +239,7 @@
 
   def testBasic(self):
     """Simple smoke test"""
-    pushimage.main(['--board', 'test.board', '/src'])
+    pushimage.main(['--board', 'test.board', '/src', '--yes'])
 
 
 if __name__ == '__main__':