Prevent caller from specifying id during ComputeNode.create()

The patch is to remove id when it was set before creating a compute
node in objects, so it can be generated automatically.

And check if the node exists before it is created.

Related to blueprint icehouse-objects.

Change-Id: Ib808643bff24b683ef330b5a1be5bc68ece30184
Merge "Prevent caller from specifying id during ComputeNode.create()"