CLEANUP: cfgparse-global: move "cluster-secret" to its own parser

CLEANUP: cfgparse-global: move "cluster-secret" to its own parser

The keyword is now registered in cfg_kws and parsed by
cfg_parse_global_cluster_secret(), making it discoverable via "-dKcfg".
Note that using ha_alert() to warn about a repetition did not make much
sense since it didn't declare a fatal error, and since it doesn't prevent
from starting, it's better turned to a warning so that's what was done
here.