Prevent caller from specifying id during Aggregate.create()

The patch is to remove id when it was set before creating an aggregate
in objects, and check if the aggregate exists before it is created.

Related to blueprint icehouse-objects.

Change-Id: I0d7c5c5da9a93e52c7fee9eee7df80d2f460593e
Merge "Prevent caller from specifying id during Aggregate.create()"