Configuration handling for RHEL7

[Thorndike]

On RHEL7: It seems that the plugin is still writing some configuration data like extra_parameters to /etc/sysconfig/docker which is basically ignored. The new way on systemd based OS seems to be drop in files in a new directory as described in

https://docs.docker.com/engine/admin/systemd/#custom-docker-daemon-options

[rothgar]

The $OPTIONS variable is set in /etc/sysconfig/docker which is then loaded by the service-override.conf file here

That's the way docker docs recommend setting it up on el too.