18562: Auto-enable preemptible when changing state to Committed.

18562: Auto-enable preemptible when changing state to Committed.

(As opposed to during any update.)

Avoids a situation where a client creates a child CR with
preemptible=false, preemptible is automatically changed to true, but
then preemptible instances are removed from config, and the client
cannot change the CR state to committed without also explicitly
clearing the preemptible flag, because the automatically-enabled
preemptible flag is not allowed.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curii.com>