Add CI job for molecule tests

Adds the following zuul job:

kayobe-tox-molecule

It runs ansible role tests using the molecule framework.

Note that we are currently running tox as root to work around issues
with applying docker group membership in the Zuul CI environment.

The ubuntu-1604 platform has been removed because the job takes a long
time to run even on one platform.

Change-Id: I8be24be828c0e124d822d7b39a02169c92b81eb3
Story: #2001637
Task: #6646