Fix flake8 errors
Test
test
File "/builds/RECIRCLE/portal/recircle/interface_openstack/test/test_image.py", line 9, in test_fields
image = Image('id', 'name', 'format')
TypeError: __init__() missing 1 required positional argument: 'size'
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Ran 11 tests in 0.016s
FAILED (errors=1)
ERROR: Job failed: exit code 1