Add env var options for the Prometheus Exporter bin scripts

The prometheus exporter bin scripts get the job done, but are quite lean and don't provide a large amount of help in setting up the exporter.

A list of things that could be improved:

• Allowing users to pass command line options through environment variables
• Support the ZK ACL environment variables
• Similar memory options to solr/bin/solr

These are just a few ideas, but a little work would go a long way here.

Previous discussion:

• docker-solr#307
• docker-solr#224