Adds a warning about the requirement to use Identity API 2.0

Object Storage is unaware of domains so ACLs cannot be enforced
when using Identity API 3.0

Change-Id: I6a1f8853e7cfcb5c4e8f789fa7fe8dfd6d3f7fdd
Closes-bug: 1299146
Merge "Adds a warning about the requirement to use Identity API 2.0"