[6.15.z] use packaged pytest, not some venv

[Satellite-QE]

Cherrypick of PR: #16694

Problem Statement

The current FAM test setup relies on being able to reach the Internet, which is "bad design"™ but also actually broken in IPv6-only environments because our lab network has IPv6, but no route to the broader Internet.

Solution

Instead of installing test dependencies via pip from the Internet, install them via dnf from the RH CDN and use them in the tests here.
There is an accompanying PR against the repo that carries the setup playbooks.

Related Issues

[Satellite-QE]

trigger: test-robottelo
pytest: tests/foreman/sys/test_fam.py
network_type: ipv6

[Satellite-QE]

PRT Result

Build Number: 9041
Build Status: UNSTABLE
PRT Comment: pytest tests/foreman/sys/test_fam.py --external-logging
Test Result : =========== 108 failed, 2 passed, 492 warnings in 2238.43s (0:37:18) ===========