Allow the `collect.py` to be overriden in crontab

Allow the `collect.py` script that is the entry point to be overriden in
the config file. This is required for the GA Collector where either
`collect.py` or `scripts/collect-content-dashboard-table.py`.
Merge pull request #17 from alphagov/allow-script-to-be-replaced

Allow the `collect.py` to be overriden in crontab