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"
Make Identity client load like the others

This does a couple of things:
* Loads the Identity client module in the same manner as the other
  'base' clients (where 'base' == 'included in the OSC repo')
* Changes the entry point group name for the base clients to
  'openstack.cli.base'.  The extension group name remains the same.
* Loads the base modules first followed by the extension modules.
  This load order ensures that the extension module commands are all
  loaded _after_ the base commands, allowing extensions to now override
  the base commands.

Change-Id: I4b9ca7f1df6eb8bbe8e3f663f3065c2ed80ce20b
Merge "Make Identity client load like the others"