Storage provides idempotent delete.

This patch defines idempotent delete (deleteIfExists) with Storage
interface, and implements it with ZookeeperObjectMapper and
InMemoryStorage. Idempotent delete is needed by clients such as
Neutron API where deletion of a model is expected to be idempotent
by the specifications.

Fix MN-2999

Change-Id: I383900a54a585f5e8025adb0b121d56633e2a213
Merge "Storage provides idempotent delete."