userstorage: replace by PyPI userstorage
    
    Remove the outdated version of userstorage
    that is stored locally at tests/storage.py
    
    The userstorage library is required
    to create the storage before running the
    tests with tox, and also in the test
    virtual environment. In order to satisfy
    this, add a requirements.txt to the root
    folder to obtain the latests userstorage
    version from PyPI, and add it as a tox
    dependency for tests.
    
    Behaviour for missing storage and folder
    structure remains as originally intended.
    
    Fixture is adapted to new style so that
    is uses setup and teardown before and
    after every test, respectively.
    
    Signed-off-by: Albert Esteve <aesteve@redhat.com>