Import missing gettext _ to fix Sphinx error
Import missing gettext _ to fix Sphinx error

We run "python setup.py build_sphinx" to generate the API documents,
however encountered a lot of "NameError: name '_' is not defined",
which will cause our api.controllers.v1.* objects documents will not
be generated by Sphinx, so we are missing these in the document page
http://docs.openstack.org/developer/ironic/py-modindex.html. Import
missing gettext _ to fix such Sphinx error with this patch.

Change-Id: Ie53d96203109b8b95b9734b58d85da41a2d011cf
Closes-Bug: #1251011

citrix-openstack-build/ironic-lib,faizan-barmawer/ironic-lib,openstack/ironic-lib,NaohiroTamura/ironic-lib,faizan-barmawer/elytics