[6.16.z] Fix upgrade errata scenario for 6.16.z

[vsedmik]

Problem Statement

Errata upgrade scenario is failing in 6.16.z for two reasons:

1. In 6.15 (pre_upgrade) new orgs are SCA-enabled by default.
2. Custom products are disabled by default.

Solution

1. Disable SCA in the pre_upgrade test (on 6.15).
2. Override AK custom repos to enabled.

Local results

Pre:

(venv) [vsedmik@localhost robottelo]$ pytest -m pre_upgrade tests/upgrades/test_errata.py -k test_pre_scenario_generate_errata_for_client[rhel7-ipv4]
================================================= test session starts =================================================
collected 6 items / 5 deselected / 1 selected                                                                                                                                                                                          

tests/upgrades/test_errata.py .                                                                                 [100%]

=============================== 1 passed, 5 deselected, 7 warnings in 612.35s (0:10:12) ===============================

Post:

(venv) [vsedmik@fedora robottelo]$ pytest -m post_upgrade tests/upgrades/test_errata.py -k test_post_scenario_errata_count_installation[rhel7]
================================================= test session starts =================================================
collected 6 items / 5 deselected / 1 selected                                                                                                                                                                                          

tests/upgrades/test_errata.py .                                                                                 [100%]

=============================== 1 passed, 5 deselected, 7 warnings in 121.94s (0:02:01) ===============================

[vsedmik]

Closing in favour of #16432