Metadata persistence in HDFS and MySQL

Partially implements the issue https://github.com/telefonicaid/fiware-connectors/issues/41

100% tests passed
Manual tests passed
Note that if you use the `if (it.hasNext()) { columnValues += ",";}` you don't need the `substring(0, columnValues.length() - 1)`filter. Moreover, using it would lead to a wrong string query composition.