Allow cron input to have comments and whitespace

As our crontab configuration files grow bigger it would be good to be
able to include comments in them to explain groups and schedules.

Not allowing blank lines or lines with just whitespace has caught a few
people out when they have been working with the crontab config files.
Now with comments they seem imperative to help keep our config files
understandable.
cronjobs should always start at main.py

The custom entry script in the cron config was a work around for dealing
with subtle variations in a collector. This is now baked into the
configuration of each query and the entry point is found and run inside of
main.py.