added ability to query clusters by the flags or rpc value

doc/man/man1/sacctmgr.1

slurm/slurmdb.h

src/common/slurmdb_defs.c

src/common/slurmdb_defs.h

src/common/slurmdb_pack.c

src/plugins/accounting_storage/mysql/as_mysql_cluster.c

src/sacctmgr/cluster_functions.c

src/sacctmgr/sacctmgr.c