Multitenancy support (HDFS and MySQL)

This PR finishes the implementation of the issue https://github.com/telefonicaid/fiware-connectors/issues/36

100% tests passed
Manually tested

Multitenancy support in HDFS is achieved by implementing the following HDFS folder schema:

    hdfs:///user/<username>/<dataset>/<entityId>-<entityType>/<entityId>-<entityType>.txt

where:

* `username` is the client username (e.g. valencia, repsol). Whitin the current version, this is the value of the `cosmos_default_username` parameter; in future releases it will be the real client username.
* `dataset`refers to the `fiware-service` header.
* `<entityId>-<entityType>` is the usual entity descriptor. In this case, a folder is created and then a file within it; the purpose is to support the creation of per entity Hive tables, and thus a specific folder is required per each entity.

Multitenancy support in MySQL is achieved by implementing the following databse schema:

    mysql:///<dataset>/<entityId>-<entityType>

where:

* `dataset` refers to the `fiware-service` header, and it is a database.
* `<entityId>-<entityType>` is the usual entity descriptor, and it is a table within the above database.