Enable localizable REST API responses via the Accept-Language header

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 exception messages to the user requested language and
return that translation from the API.

Partially implements bp user-locale-api.

Change-Id: I63edc8463836bfff257daa8a2c66ed5d3a444254
Merge "Enable localizable REST API responses via the Accept-Language header"