Add support for API message localization

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 now
possible to use the resolved language to translate an exception message
to the user requested language and return that translation from the API.

Partially implements bp user-locale-api

Change-Id: Ib2c8360372996d53b50542df54a52d92b07295ca