12199: Keep SLURM's node properties up to date.

12199: Keep SLURM's node properties up to date.

Change the semantics of ComputeNodeUpdateActor.sync_node so it gets
called every time a new Arvados node record appears, even if hostnames
match. The base actor's implementation now compares hostnames itself
before calling the cloud driver.

This allows the slurm update actor's sync_node method to sync SLURM
state periodically, even if hostnames don't go out of sync.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tclegg@veritasgenetics.com>