Add way to group exceptions in DependencyTaskGroup

DependencyTaskGroup currently aborts execution if any exceptions are
raised.  This commit provides a way of aggregating all the exceptions
during execution and re-raising them only at the end.

blueprint stack-check

Change-Id: I6af314cc112f9286c8b38739870c6fa8b4ba9b5a
Fix comparison with singletons

Many plases in tests code use assertEqual(AA, None),
the same with True and False and negatives of such comparisons.

As comparison to singletons must use `is` operator, appropriate usage
is assertIs, assertIsNot, assertIsNone, assertIsNotNone

These assertions are provided by the `testtools` package,
so this change is not breaking backward compatibility with Python 2.6

This second patch inlcudes the other half of the files touched by these
changes to ease the review.

Change-Id: I9965ae158f3e0dc49b93b9357a32a302ac7dc735
Closes-Bug: #1259023
Merge "Fix comparison with singletons"
Merge "Add way to group exceptions in DependencyTaskGroup"