identity: check whether current stream is meant to auto-update

[lucab]

At the moment Zincati tries to always auto-update the OS, regardless of its stream.

This is fine on production streams. There are however few cases where this doesn't make much sense, for example mechanical and development streams.

It would be good to expose a configurable list of streams for which auto-updates are enabled. Default configuration would allow production streams only.

Raw plan:

• Add a stream_whitelist configuration entry (HashSet<String>)
• Add logic for override-merging stream entries from multiple configuration fragments
• Add a default TOML configuration fragment with [ "stable", "next", "testing" ]
• Fast-forward update-agent state-machine to EndState if the current stream is not in the whitelist

Blockers:

• identity: drop hardcoded stream label #55

[jlebon]

See coreos/fedora-coreos-tracker#163 (comment) for a different approach. To directly address this specific issue's title:

Zincati would be configured appropriately for each individual stream, so it wouldn't have to care about specific stream names.

[lucab]

Closing in favor of #62.