identity: drop hardcoded stream label

[lucab]

Zincati needs to introspect its own Fedora CoreOS stream label at runtime, in order to request the relevant update-graph to the Cincinnati backend. This information is a read-only property of the booted deployment, and it should be exposed by rpm-ostree.

We are currently using an hardcoded value at

zincati/src/identity/mod.rs

Line 129 in 025453b

// TODO(lucab): read this from os-release.

We should replace it with proper stream label sourcing. This is currently blocked on coreos/fedora-coreos-tracker#163.