Clarify FactoryReset os version behavior

[LimeHat]

Today FactoryReset service uses factory_os flag in StartRequest message to initiate a "rollback" to the factory OS version.

Is there a specific requirement for what should happen when this flag is not set?

• Should the current OS version always be presumed?
• What if an implementation supports defining a "golden image" version (specific version) to reset to (e.g. via CLI in advance, or by some other means); is there a way to incorporate this into FactoryReset?