Add support for API message localization

Using the lazy gettext functionality from oslo gettextutils,
it is possible to use the Accept-Language header to translate
an exception message to the user requested locale and return
that translation from the API.

Implements bp user-locale-api

Change-Id: I48de5e681827305eef60c7a77e8d3091f9d64be0
Co-authored-by: Mathew Odden <mrodden@us.ibm.com>
Co-authored-by: Ben Nemec <openstack@nemebean.com>
Merge "Add support for API message localization"