Make missing imports flag in hacking settable.
Make missing imports flag in hacking settable.

Change-Id: Ib1ada320588eb42370c9fa2c0eff4eb888258ea5

hyakuhei/cleantox,openstack-dev/hacking,emonty/oslo-hacking,openstack-dev/hacking,zancas/hacking,hyakuhei/cleantox,emonty/oslo-hacking,zancas/hacking