[region_mover.rb] Add option to exclude list of hosts on unload instead of just assuming the source node.

Currently, the region_mover.rb script excludes a single host, the host offloading data, as a region move target. This essentially limits the number of machines that can be shutdown at a time to one. For larger clusters, it is manageable to have several nodes down at a time and desirable to get this process done more quickly.

The proposed patch adds an exclude file option, that allows multiple hosts to be excluded as targets.