onlinecheck changes

[andrew-naish]

Use cron instead of while sleep

It's what cron is made for after all.
There is already a baked in 15min interval, I can use that.
https://devopsheaven.com/cron/docker/alpine/linux/2017/10/30/run-cron-docker-alpine.html

Replace ONLINECHECK_DELAY with ONLINECHECK_QUERYURL

I think I should remove the option to easily set the delay and instead add the option to configure the query URL.
After switching to cron this will be harder to set anyway.
The default is and will remain https://www.google.co.uk if unset.