Updates to the HDInsight job submission

This addresses #909, the failure to submit to HDInsight from computers that aren't running Windows. The main issue was in `HDInsightClasspath`, which assembled a classpath using the path separator character of the machine submitting the job. This is unsafe, as the classpath for HDInsight needs always to be assembled using ";".
Merge pull request #910 from Microsoft-CISL/weimer_909

Updates to the HDInsight job submission. Self-merge as I validated this on windows and linux
Merge branch 'master' of https://github.com/Microsoft-CISL/REEF into MLSS