Static messages are not localized when requested

Messages created at import-time are not being translated when
the request is made with Accept-Language.

Note that messages created a run-time are being translated.

Since not many messages are translated at this point, to recreate
can set a translated message yourself. For example, update
keystone/keystone/locale/it_IT/LC_MESSAGES/keystone.po:

 #: keystone/exception.py:101
 msgid "The request you have made requires authentication."
 msgstr "Wonderful Italian rendition of this"

before running `python setup.py compile_catalog`

Then run Keystone and request a resource that requires authentication.

curl -s -H "Accept-Language: it" http://localhost:5000/v3/auth/tokens | python -mjson.tool

{
    "error": {
        "code": 401,
        "message": "The request you have made requires authentication.",
        "title": "Unauthorized"
    }
}

-- this should return

{
    "error": {
        "code": 401,
        "message": "Wonderful Italian rendition of this",
        "title": "Unauthorized"
    }
}