Revert recent change to ComputeNode

This reverts the object-related parts of 4c21f4a. The problems with
it are:

- It didn't bump the object version
- It introduced a JSON blob string attribute to the object,
  which is unversioned structured data, which can't be
  checked or guaranteed during upgrades.

The first one, of course, breaks live upgrade because an older
node receives fields it doesn't know about, but should based on
the version.

Offending change I75e339c2a2827875078c937e57c37e85d3ad1852

Change-Id: Ie280dc6fe27e48aea9939131c9e695c60b132414
Merge "Revert recent change to ComputeNode"