Add oslo.i18n as a dependency

Add i18n in requirements.txt

implements bp add_i18n

Change-Id: I84ecd16696593414739c52ee344b8a1c9868941a
Create message variables for exceptions

Instead of inline messages, let's create variables instead, as
it's easier to find strings, and mark them for translation.

Change-Id: Ibbcfdbc59d12a0cb4af50f73043d3ff7f3c76f99
Add i18n module to openstackclient

Based on the information available at:
http://docs.openstack.org/developer/oslo.i18n/usage.html

implements bp i18n

Change-Id: Ie44f95dcbf192736991f88d92773f0dc2e20fa64
Merge "Add oslo.i18n as a dependency"
Merge "Create message variables for exceptions"
Merge "Add i18n module to openstackclient"