ListenCoresBucketsRatio support should be added to apache config options

In the big 4 archive servers we've been working with some manual cowboys to config options as we've worked to mitigate the performance issues and outages we've seen from those nodes.

One such option added was setting ListenCoresBucketsRatio in the mpm_event.conf, which we should probably add to the charm and make tunable as apache_listencoresbucketsratio?

Per https://httpd.apache.org/docs/2.4/mod/mpm_common.html the default value would be 0 (disabled).