Make program enrolment batch size configurable

[kneckinator]

To ensure that the OpenSPP can be tuned better to a given hardware, the batch size of the enrolment job should be configurable.
Today, it is hard-coded to 10,000.

Making this configurable will ensure that we can run multiple batches in parallel without causing MemoryErrors, while at the same time benefitting from good throughput and allowing enrolments into multiple programs to happen in parallel.

The configuration should be done through an environment variable, which is read as the server starts. This allows for DevOps to tune this setting without having access to the OpenSPP web interface, and without redeploying any files.