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

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

https://github.com/GoogleCloudPlatform/testgrid/blob/651499da486f08c6ba6fcfa7efb1535318c22fa5/cmd/config_merger/main.go#L93

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.

https://github.com/GoogleCloudPlatform/testgrid/blob/651499da486f08c6ba6fcfa7efb1535318c22fa5/cmd/config_merger/main.go#L111-L125

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