Hitrate based file provision mistakenly dependent on site instead of cache

[HerrHorizontal]

When the hitrate for a job's input files is computed to make the decision, whether the files are provided by a cache, the drone's sitename is used as a key to check whether it provides the input files instead of looping over all the storages on the according site: https://github.com/tfesenbecker/lapis/blob/dd876d8b0f6519f105c56af8ef735a707df0ae95/lapis/connection.py#L184-L190

This is part of the issue #6 and should also be considered when tackling issue #3.

[HerrHorizontal]

@tfesenbecker proposes:

 hitrate = sum( 
     [ 
         file["usedsize"] * file["hitrates"].get(cache.name, 0.0) 
         for file in requested_files.values()  for cache in self.storages[drone.sitename]
     ] 
 ) / sum([file["usedsize"] for file in requested_files.values()])