Sync request_utils module from oslo-incubator

The request_utils module hasn't had a sync done since shortly
after the module was added.  This patch brings us up to date
with the latest oslo-incubator code.

This sync is needed to enable removal of the old gettextutils
and i18n code.

Current HEAD in OSLO:
---------------------
commit 36b0e8570b449129d6d474c03b02ceb62edb78df
Date:   Thu Dec 11 11:27:08 2014 +0100
We shouldn't replace `oslo-incubator` in comments

Changes being merged with this patch:
---------------------
5d40e143 - Remove code that moved to oslo.i18n
39625e18 - Set pbr 'warnerrors' option for doc build

Change-Id: Ib5b75d6ff38ff6f9571a0fdc30e854e046e66dd7
Merge "Sync request_utils module from oslo-incubator"