Disallow empty strings in request

Most parameters in request should not be empty strings. We add
check for emptiness and additional keyword argument to String schema
type class to allow to disable this check.

Change-Id: I959614a44fc183938cad155dc0b2d9e4bda35817
Delete keypairs when deleting their owner

Change-Id: Ia4223a2e22eb97f8e8b5fef4dd846eabc9f43c8e