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: I446fdf4b9843155e0c818c271114b1e2e4f43b64
Merge "Add support for API message localization"