Remove unused common.base module

Base module is already replaced with a module from oslo common code,
but was not removed. This patch must fix this.

Change-Id: I8f5c4da8884db4024d47ca8d1a80dbb648922a31
Fix comparison with singletons

Many places 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

Change-Id: I232136ce49afdb8bf78b42221454540471d12c23
Closes-Bug: #1259023
Merge "Remove unused common.base module"
Merge "Fix comparison with singletons"