Add ids handler

[gigerdo]

This change implements support for lists of secret values in fleet integration policies. There are two ways a secret value can be defined:

• Just a simple string
  • "password": "secret" <-> "password": {"id": "Zgr3ZJgBz9OcmZgWR1vo", "isSecretRef": true}
• A list of strings
  • "hosts" : ["root:test@tcp(127.0.0.1:3306)/"] <-> "hosts": {"ids": ["Zgr3ZJgBz9OcmZgWR1vo], "isSecretRef": true}

The code now correctly maps between both cases. (It used to only handle the simple string)

The bug report actually contains a second bug that shows when using vars_json = "{}",. Since the API always responds with a nil value, it generates a diff between the config and the actual state. The solution here is to map nil to "{}" if the local config contains "{}".