Body for CE Communicator queue message specifies both _id and identifier

[Archer6621]

See /ce_integration/ce_communicator.py. There is a difference in the way the ID is retrieved from the JSON body of the input queue (mq_ce_communicator) for some task depending on whether it is being created or being updated. This difference is probably a left-over and should be fixed/unified. The proposal is to use _id, since we use that as name for the ID field in other places as well.