Get completely rid of contextlib.nested

Get completely rid of contextlib.nested

`contextlib.nested` is deprecated since Python 2.7 and incompatible with
Python 3. This patch removes all its occurences by using the helper
script at [1].

This is a necessary step to allow us running all unit tests with
Python 3 (not just a small subset as it is done now).

[1]: https://github.com/adrienverge/context_unnester

Change-Id: I8d1de09ff38ed0af9fb56f423a2c43476408e0fb
Blueprint: neutron-python3
Closes-Bug: #1428424