incus: Improve completion for config keys #3015

Job	Run time
Code (oldstable)	5m 19s
Code (stable)	5m 18s
Code (tip)	5m 10s
Client (oldstable, ubuntu-latest)	48s
Documentation	5m 17s
Client (oldstable, macos-latest)	47s
Client (oldstable, windows-latest)	1m 56s
System (stable, cluster, dir)	5m 16s
Client (stable, ubuntu-latest)	52s
System (stable, cluster, btrfs)	5m 16s
Client (stable, macos-latest)	49s
System (stable, cluster, lvm)	5m 17s
Client (stable, windows-latest)	1m 3s
System (stable, cluster, zfs)	5m 17s
System (stable, cluster, ceph)	5m 16s
System (stable, cluster, random)	5m 15s
System (stable, standalone, dir)	5m 16s
System (stable, standalone, btrfs)	5m 16s
System (stable, standalone, lvm)	5m 15s
System (stable, standalone, zfs)	5m 15s
System (stable, standalone, ceph)	5m 16s
System (stable, standalone, random)	5m 16s
System (oldstable, cluster, dir)	5m 16s
System (oldstable, standalone, dir)	5m 18s
System (tip, cluster, dir)	5m 15s
System (tip, standalone, dir)	5m 17s
	1h 51m 36s

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