Backward compat for allowed_origin
Backward compat for allowed_origin

In Ie2e57b76717604f701daa16ebf8ffa8c06835e3c, we switched
from StrOpt to ListOpt for allowed_origin configuration
option. For example ironic uses set_override to set
the value to a string in test cases. We should allow
the older behavior for some time to allow ironic
test case TestCORSMiddleware to switch over.

Change-Id: Ifac325871d89d9e85c6e74c05ba6e004aeb741b9

openstack/oslo.middleware