OpenStack empty config handling broken

[yasminvalim]

Problem

After adding single-stack IPv6 support in PR #1909, the fcos.ignition.misc.empty kola test started failing. The test verifies that not injecting SSH keys into Ignition works on FCOS, which requires that when no config is provided, Ignition receives empty bytes (not an empty JSON object).

When dual-stack IPv4/IPv6 support was added, the code was changed to validate fetched data by unmarshalling it (see this change). However, the code then marshals the unmarshalled types.Config struct back to JSON before returning it.

Error Logs

--- FAIL: fcos.ignition.misc.empty (630.47s)
04:03:30          harness.go:106: TIMEOUT[10m0s]: SSH unsuccessful within allotted timeframe for 2be6b652-338a-4e7e-882a-9c43e4104138.

from the console log:
ignition[677]: failed to fetch config: invalid config version (couldn't parse)
ignition[677]: failed to acquire config: invalid config version (couldn't parse)
ignition[677]: Ignition failed: invalid config version (couldn't parse)

[yasminvalim]

PR: #2177