[hbck2] Schedule SCP for all unknown servers

Add a command to HBCK2 that schedules SCPs for unknown servers. This command is useful when a new installation of HBase is pointing to a previously used HBase root directory where the hbase:meta table is populated. This will contain references to RegionServers from the previous installation and SCP will not be created against these servers and the regions won't be assigned to the new server.

The scheduleRecoveries command in HBCK2 could be used to initiate a SCP one-by-one for the unknown servers, however, collecting the list of servers could be difficult on a large cluster.

I'm proposing to add a command to HBCK2 that will schedule SCPs for all the unknown servers in the cluster.