modify hacking.py to not choke on the def of _()
modify hacking.py to not choke on the def of _()

recent openstack-common brought in it's own def of _() which trips
up our checking that all _ calls contain a message string.

This explicitly excludes the definition of this in function from
that check.

Fixes LP Bug #1020792.

Change-Id: If647d77a5ae5560935bb2e032331c34b896b8614

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