Add max-instances and concurrency options

[glasnt]

Only adds options if defined by user (otherwise doesn't pass a default value, allowing downstream systems to specify)

Resolves #227

Addresses request in #108, but doesn't scale the solution as suggested.

Tested in isolation, pending automated testing

[jamesward]

Thanks! This looks good to me. Unfortunately we don't have any automated integration tests. So I'll let you merge this after when you are ready / feel good with sufficient manual testing.