Add prefix to storage config

[joao-zanutto]

As discussed in #56, the storage.prefix config should be added to work with both storage types directory and s3, meaning that s3.prefix will be deprecated.

Concerns:

• Will storage.directory.path config remain the same or should it be removed to make place to storage.prefix as well?
  • if so, which will the final storage path given the user configuration defines both?
    • {{ storage.prefix }} / {{ storage.directory.path }} or
    • {{ storage.directory.path }} / {{ storage.prefix }}

[wwoytenko]

The easiest one is storage.prefix. We could provide a path prefix as a parameter in the constructor