DRILL-1133: Default cp and dfs storage plugin instances do not appear in Drill Web UI after installing Drill.

* Modified StoragePluginRegistry to scan for ALL 'bootstrap-storage-plugins.json' files in the classpath instead of only the first one.
* Moved 'hbase' and 'hive' plugin instance definitions to their own module from 'java-exec'.
* Renamed the test storage plugin instances to 'dfs_test' and 'hive_test' and updated the test cases to reflect this change.