Major refactoring of backup slurmctld logic

Configuration parameters "ControlMachine", "ControlAddr", "BackupController"
   and "BackupAddr" replaced by an ordered list of "SlurmctldHost" records
   with the optional address appended to the name enclosed in parenthesis.
   For example: "SlurmctldHost=head(12.34.56.78)". An arbitrary number of
   backup servers can be configured.