Doc consistency between help and confirm text

The delete-all command's help text did not match the confirm
text for the delete-all command.  The help now matches the
confirm text and is gramatically correct

Change-Id: If9c37958192dbdd2de61e74fe95aad2d8a170ca3
Reviewed-on: https://review.openstack.org/18685
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins