Trivial: ignore openstack/common in flake8 exclude list
Trivial: ignore openstack/common in flake8 exclude list

The directory openstack/common doesn't exist any more,
so remove it from flake8 exclude list.

Change-Id: I59e0b4c4c3ca5345260d6c0f2ef3a3a27cc3535b

NaohiroTamura/python-ironicclient,NaohiroTamura/python-ironicclient,openstack/python-ironicclient,openstack/python-ironicclient