Allow stroom-log-sender to use the stack .env file in preference to crontab.env

[stroomdev10]

Has been started already.. https://github.com/gchq/stroom-resources/commits/use-supercronic

[at055612]

The issue is that normal cron runs with a clean environment so it is not possible to access env vars passed into the container. Also cron is not best suited to running in docker as it runs as a daemon not a foreground process. Using supercronic https://github.com/aptible/supercronic means we can read env vars passed to the container.

Some work was done on this already, see branch linked above. It needs updating from 6.0 or master depending on where it came from (probably 6.0) and testing. I seem to remember there was some issue with the format of the log output, as i think supercronic wraps everything in its own log format.