Metadata persistence in HDFS and MySQL

Partially implements the issue https://github.com/telefonicaid/fiware-connectors/issues/41

100% tests passed
Manual tests passed
It is the magic I've used for deleting the whitespaces between tags. This way:
    <tag1>1</tag1>           <tag2>2</tag2>
Becomes:
    <tag1>1</tag1><tag2>2</tag2>