Add oslo.i18n as dependency

oslo.i18n provides the i18n functions that were provided by
oslo-incubator's gettextutils module. Once Keystone is changed to
use oslo.i18n the gettextutils module can be removed.

Change-Id: Ie9db1c25ba0601258fb9a493cbc75b0ff205d0d6
Merge "Add oslo.i18n as dependency"