Replaces the unittest2 base module with testtools.
Adds py36 unit test target in tox.ini, called from zuul.
Updates unit tests to pass when run in both py27 and py36.
Add sample CLI unit tests for fw update strategy based on
a mixin for update strategy.
Story: 2007875
Task: 40674
Change-Id: Ia1b94110a53d48249aed76c94b07f8ffc2946bb3
Signed-off-by: albailey <Al.Bailey@windriver.com>