Fix F402 pep8

Fix:
./manila/api/contrib/used_limits.py:45:27: F402 import 'quota' from line
19 shadowed by loop variable

Rename the loop variable.

Remove F402 from ignore list since it now passes.

Change-Id: Iaa1f882d0ce1843496283a922b701860746995a2
Merge "Fix F402 pep8"