V1.1: Add id property to messages in claims

The id property should be also added to messages returned from the
claims endpoints: claim messages and get claim.

Change-Id: Ida02aa4e1c794adb3ae7ea26cdd250ae45cac5bd
Partially-implements: blueprint api-v1.1-response-document-changes
Merge "V1.1: Add id property to messages in claims"