(merge fixup) cache maintenance script

Ibc7ec0a90a130fb1c834fd632af7872f1325b3e0 altered
CachedRequest._cachefile_path() so it was not usable for cache.py
introduced in I727af9b6fa10d2231e5d777a1bc6f8602ca03f50.

Both being committed at the same time caused cache.py to fail
for directories other than the one which CachedRequest uses.

Change-Id: I36360fd041da952375bc88a4599a21451878baa7