paygen: Make PaygenPayload arguments optional.

Sometimes we need to create the PaygenPayload object without needing to
sign or verify the payload. Or maybe we have some other objective for
the class itself. So make these two parameters optional with default
values.

BUG=chromium:872441
TEST=payload-tryjob
TEST=unittest

Change-Id: I83153e8b84bec62254f12749c4e7ae3d96a03c40
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1651281
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Nicolas Norvez <norvez@chromium.org>
Commit-Queue: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
diff --git a/lib/paygen/paygen_payload_lib.py b/lib/paygen/paygen_payload_lib.py
index 4fca40e..b52c953 100644
--- a/lib/paygen/paygen_payload_lib.py
+++ b/lib/paygen/paygen_payload_lib.py
@@ -71,7 +71,8 @@
   _KERNEL = 'kernel'
   _ROOTFS = 'root'
 
-  def __init__(self, payload, work_dir, sign, verify, private_key=None):
+  def __init__(self, payload, work_dir, sign=False, verify=False,
+               private_key=None):
     """Init for PaygenPayload.
 
     Args:
@@ -881,7 +882,7 @@
   # We need to create a temp directory inside the chroot so be able to access
   # from both inside and outside the chroot.
   with chroot_util.TempDirInChroot() as work_dir:
-    PaygenPayload(payload, work_dir, sign, verify).Run()
+    PaygenPayload(payload, work_dir, sign=sign, verify=verify).Run()
 
 
 def GenerateUpdatePayload(tgt_image, payload, src_image=None, work_dir=None,
@@ -905,9 +906,9 @@
   payload = gspaths.Payload(tgt_image=tgt_image, src_image=src_image,
                             uri=payload)
   with chroot_util.TempDirInChroot() as temp_dir:
-    work_dir = work_dir or temp_dir
-    paygen = PaygenPayload(payload, work_dir, private_key is not None, check,
-                           private_key=private_key)
+    work_dir = work_dir if work_dir is not None else temp_dir
+    paygen = PaygenPayload(payload, work_dir, sign=private_key is not None,
+                           verify=check, private_key=private_key)
     paygen.Run()
 
     # TODO(ahassani): These are basically a hack because devserver is still need