Add cache test

This test attempts to validate all cache entries, at the end of
the build as a sanity check.

DataSite cache entries were failing to load due to having a
different __init__ signature from APISite.

CacheEntry.params renamed to _params to reflect changes in the
Request object.

Change-Id: I64b685776ad10611c91631e1937613e846972824
Merge "Add cache test"