Updates BalancerRequest to run the balancer in dryRun mode. In this mode, the balancer will
try to find a plan but WILL NOT execute any region moves or call any coprocessors. You can
run in dryRun mode regardless of whether the balancer switch is enabled or disabled, but
dryRun mode will not run over an existing request or chore. Dry run is useful for testing out
new balance configs. See the logs on the active HMaster for the results of the dry run.