Run slow tests parallely

We started the slow test run serially to avoid
any timeout issue in parallel run. But serial run
takes lot of time and end up with job timeout.

tempest-slow-parallel job exepriment running the slow
tests in parallel and its not so bad. Only 1 timeout
in last 50 run
- https://zuul.opendev.org/t/openstack/builds?job_name=tempest-slow-parallel&skip=0

Let's try running slow tests in parallel in tempest-slow-py3
job itself and if it make situation worst then we can revert
it to run serially.

Change-Id: I1e32e7a07018a05b54515d9e6e6c8bc2fc7fbdff
2 files changed