Delete *openstack/common* in flake8 exclude list
Delete *openstack/common* in flake8 exclude list

There is not *openstack/common* directory in mistral-dashboard,
we should drop it for improving searching efficiency.

Change-Id: I0f41e65e78e082cdd0f637ab07fb872b1b195a86

openstack/mistral-dashboard,openstack/mistral-dashboard,openstack/mistral-dashboard