Activate pep8 check that _ is imported

Removed the specification in tox.ini that _ is a builtin so that
it will no longer assume that _ does not need to be imported.

The correct import is:

    from cinder.i18n import _

Activating this check did not flag any violations in cinder.

Change-Id: Ifc789514d2462c059eb9f003fac288e751331ded
Merge "Activate pep8 check that _ is imported"