rocksdb_update_cf_options is useless when specified in my.cnf or on command line.

This seems due to the fact that SYSVAR 'validate' and 'update' callbacks are not called on startup when an option is specified either on the command line or in a .cnf file.  For MyRocks, we can either manually perform the validation and setting from the plugin init function, or, mark the option with PLUGIN_VAR_NOCMD_OPT.