Make systemd>=207 hosts use /etc/sysctl.d/99-salt.conf

As of systemd 207, /etc/sysctl.conf is now ignored, and
/etc/sysctl.d/*.conf are used. This commit makes linux hosts check to
see if they are running systemd, and for new enough versions it uses
/etc/sysctl.d/99-salt.conf for the default config file.

Resolves #7351.