Fix the download() function in file_utils.py, so it provides a way to specify both dirname and filename arguments.
At the moment, there is just "dest" which can be ambivalent sometimes.

The expected behavior is that dirname is always required. If the directory does not exist, it gets created. filename is either provided as an argument, or if the argument is set to None (default value), then it is infered from the URL in the same way as at the moment.

Once this is fixed, please also adjust all uses of this method in taskotron and tasks. At the moment of creation of this ticket, it is only used in libtaskotron/koji_utils.py/get_nvr_rpms()