Memoizing decorator (memoized) for data driven tests.

* Implements memoized deorator to add cacheing and logging to
  classmethods that are called outside of the scope of the base
  test fixture (such as DatasetListGenerators).
* Prevents decorated methods from making multiple redundant
  api calls via caching (memoization).
* Prevents 'log handler not found' messages by adding handler to
  root logger for duration of decorated method to properly log those
  messages (for when those calls are made before logging is
  initialized).

Change-Id: Ia7d3a74a38b7193748fe91ff51c1a0a73e4b3ca5
Merge "Memoizing decorator (memoized) for data driven tests."