RFR: 8217338: [Containers] Improve systemd slice memory limit support

[Severin Gehwolf]

On Fri, 2019-03-22 at 14:25 -0400, Bob Vandette wrote:
> Could you maybe combine subsystem_file_contents with subsystem_file_line_contents
> by adding an additional argument?

Done: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8217338/05/webrev/

Thanks,
Severin