Validates that 'value' is file-like and readable.

def validate_readable(option, value):
    """Validates that 'value' is file-like and readable.
    """
    if value is None:
        return value
    # First make sure its a string py3.3 open(True, 'r') succeeds
    # Used in ssl cert checking due to poor ssl module error reporting
    value = validate_string(option, value)
    open(value, 'r').close()
    return value