Config Merger only reads configuration on startup; should be more often

[chases2]

The Config Merger is a controller that takes multiple configurations and merges them together.
It requires a list of configurations to merge together.

testgrid/cmd/config_merger/main.go

Line 93 in 651499d

list, err := merger.ParseAndCheck(file)

It then periodically reads those configs and merges them together. Changes in the config are picked up in this loop, but changes to the config list are not.

testgrid/cmd/config_merger/main.go

Lines 111 to 125 in 651499d

	updateOnce := func(ctx context.Context) {
	start := time.Now()
	ctx, cancel := context.WithTimeout(ctx, 10*time.Minute)
	defer cancel()
	log.Info("Starting MergeAndUpdate")
	err := merger.MergeAndUpdate(ctx, client, list, opt.skipValidate, opt.confirm)
	cycle.Set(int64(time.Since(start).Seconds()), "config_merger")
	if err != nil {
	log.WithError(err).Error("Update failed")
	errors.Add(1, "config_merger")
	return
	}
	successes.Add(1, "config_merger")
	log.Info("Update successful")
	}

This has caused issues when the config is changed, and Config Merger silently ignores those changes until it's rebooted. See GoogleCloudPlatform/oss-test-infra#919

Additional logic to determine if the config list has changed and to either log this event or do less work if it hasn't changed would also be nice.