Remove use of contextlib.nested

Remove use of contextlib.nested

The contextlib.nested call has been deprecated
in Python 2.7. This causes DeprecationWarning
messages in the unit tests.

There are also known issues with contextlib.nested
that were addressed by the native support for
multiple "with" variables. For instance, if the
first object is created but the second one throws
an exception, the first object's __exit__ is never
called.

Since Cinder no longer supports 2.6 we can remove
the use of these contextlib.nested calls.

Added hacking check to catch if any new instances
are added to the codebase.

Note: line continuation markers (e.g. '\') had to
be used or syntax errors were thrown. While using
parentheses is the preferred way for multiple line
statements it is not a requirement.

Partial-Bug: 1428424
Change-Id: I7bb7d201d31ff239be3402fb64e5f202ede019b0