[TEST] Prevent usage of System Properties in the InternalTestCluster

All settings should be passes as settings and the enviroment should not
influence the test cluster settings. The settings we care about ie.
`es.node.mode` and `es.logger.level` should be passed via settings.
This allows tests to override these settings if they for instance need
`network` transport to operate at all.
I think it might make sense to do the same in `ExternalTestCluster`, so that we state that network mode is required.