devserver: eliminate mutable default arguments

Fixes a potential error due to Python's declaration-time binding of
default arguments. In essence, the rule of thumb is not to use default
values that are mutable objects.

BUG=None
TEST=Unittests; devserver starts and runs

Change-Id: Ib5a45f2f500c6d73eb94d0342f834a90097cb058
Reviewed-on: https://gerrit.chromium.org/gerrit/35416
Reviewed-by: Chris Sosa <[email protected]>
Commit-Ready: Gilad Arnold <[email protected]>
Tested-by: Gilad Arnold <[email protected]>
1 file changed