Add --allow-user-managed (modeled on bin/solr start --user-managed) param

[epugh]

Description

Turns out you can use solr-orbit with non cloud:

solr-orbit run \
  --pipeline=benchmark-only \
  --target-host=localhost:8983 \
  --kill-running-processes \
  --workload=nyc_taxis \
  --test-mode \
  --include-tasks="check-cluster-health,index"

This introduces a switch that, when you combine it with smart use of --include-tasks allows you to use it with standalone, but with caveats.

Testing

• New functionality includes testing
  unit test, and then manually.

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