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.