Enable F402 checks and fix violations

This change enables F402 hacking check and fixes all violations found.

[F402] import module from line N shadowed by loop variable

Basically this rule tries to avoid loop variables using the same name as
imported module, for example:

    from cinder import volume
    ...

    for volume in volumes:
        ...

Change-Id: Ie3010616ca87fda113d452fe684e6cff3d5fd978
Merge "Enable F402 checks and fix violations"