Change Identity API default version to 3

Devstack now issues all v3 OSC commands for our CI, and since v3
has everything v2 has, we should be able to switch to v3.

Furthermore, most OSC users were initially  using OSC since they
were looking to exploit v3 keystone capabilities.

Change-Id: If7de86946f6dd0f148aa5f526155cccf90adfcc3

Author: Steve Martinelli

openstackclient/identity/client.py

@@ -21,7 +21,7 @@
 
 LOG = logging.getLogger(__name__)
 
-DEFAULT_API_VERSION = '2'
+DEFAULT_API_VERSION = '3'
 API_VERSION_OPTION = 'os_identity_api_version'
 API_NAME = 'identity'
 API_VERSIONS = {