Modify the help message for getting identity extensions
Modify the help message for getting identity extensions

'keystone discover' is no longer valid to get identity
extensions, this is to change to use 'openstack extension
list --identity' in help message in config.py

Change-Id: Icfdf0fc9df67f697a52b82b55c4e4a3f63f447d7

cisco-openstack/tempest,cisco-openstack/tempest,masayukig/tempest,masayukig/tempest,openstack/tempest,openstack/tempest