Changes to DistributedTestRule

Hi there Apache Devs, I've committed changes to the DistributedTestRule, which would allow a DUnit test to create less than the default 4 VM's. So if you use the DistributedTestRule in tests that don't require the default 4 vm's you can now change those tests to not spin up the unused VM's.

Build for version 1.7.0-build.868 of Apache Geode failed.

= The build job for Apache Geode version 1.7.0-build.868 has failed. Build artifacts are available at: http://files.apachegeode-ci.info/builds/1.7.0-build.868/geode-build-artifacts-1.7.0-build.868.tgz

[Spring CI] Spring Data GemFire > Nightly-ApacheGeode > #897 was SUCCESSFUL (with 2376 tests). Change made by John Blum.

--- Spring Data GemFire > Nightly-ApacheGeode > #897 was successful. --- Scheduled with changes by John Blum. 2378 tests in total. https://build.spring.io/browse

Geode unit tests completed in 'develop/FlakyTest' with non-zero exit code

Pipeline results can be found at: Concourse: https://concourse.apachegeode-ci.info/teams/main/pipelines/develop/jobs/FlakyTest/builds/463