Add support for API message localization

Add support for doing language resolution for a request,
based on the Accept-Language HTTP header. For example,
an HTTP client can receive API messages in Chinese even
if the locale language of the server is English.

The underscore (_) method is initialized so that it returns
openstack.common.gettextutils.Message objects. The locale
of these Message objects is set when exceptions with which
they are associated are raised in the context of an HTTP
request.

docImpact
Partially implements bp i18n-messages

Signed-off-by: Fei Long Wang <flwang@cn.ibm.com>
Signed-off-by: John Warren <jswarren@us.ibm.com>

Change-Id: I352cda57fe119022c59c6c813b5c8053765b2d3c