Add support for API message localization

Add support for doing language resolution for a request, based on the
Accept-Language HTTP header.

Using the lazy gettext functionality from oslo gettextutils, it is
possible to use the resolved language to translate an exception message
to the user requested language and return that translation from the API.

Co-authored-by: Luis A. Garcia <luis@linux.vnet.ibm.com>
Co-authored-by: Mathew Odden <mrodden@us.ibm.com>

Implements bp user-locale-api

Change-Id: Id8e92a42039d2f0b01d5c2dada733d068b2bdfeb
Merge "Add support for API message localization"