scylla_post_install.sh: generate memory.conf for CentOS7
scylla_post_install.sh: generate memory.conf for CentOS7

On CentOS7, systemd does not support percentage-based parameter.
To apply memory parameter on CentOS7, we need to override the parameter
in bytes, instead of percentage.

Fixes #6783

scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla