Previously, if any resources were found to have drifted, the JSON plan
output would include a drift entry for every resource in state. This
commit aligns the JSON plan output with the CLI UI, and only includes
those resources where the old value does not equal the new value---i.e.
drift has been detected.
Also fixes a bug where the "address" field was missing from the drift
output, and adds some test coverage.
Alisdair McDiarmid