The configuration knobs for root keyring garbage collection are present in the
consumer and present in the user-facing config, but we missed the spot where we
copy from one to the other. Fix this so that users can set their own thresholds.
The root key is automatically rotated every ~30d, but the function that does
both rotation and key GC was wired up such that `nomad system gc` caused an
unexpected key rotation. Split this into two functions so that `nomad system gc`
cleans up old keys without forcing a rotation, which will be done periodially
or by the `nomad operator root keyring rotate` command.
