[6.15.z] Update HTTP Proxy e2e test cases

[Satellite-QE]

Cherrypick of PR: #14215

Problem Statement

During the HTTP proxy component evaluation PX/SD/QE agreed to make several changes. This PR addresses those related to the E2E tests, in particular:

• extends test_positive_end_to_end to sync other content type repos via HTTP proxy
• decreases CaseImportance of some

Some other changes improve docstings - remove redundant fields and adds more information.

Related Issues

SAT-23276

[Satellite-QE]

trigger: test-robottelo
pytest: tests/foreman/api/test_http_proxy.py tests/foreman/cli/test_http_proxy.py

[Satellite-QE]

PRT Result

Build Number: 5935
Build Status: UNSTABLE
PRT Comment: pytest tests/foreman/api/test_http_proxy.py tests/foreman/cli/test_http_proxy.py --external-logging
Test Result : ===== 1 failed, 9 passed, 2 deselected, 476 warnings in 3246.53s (0:54:06) =====

[Satellite-QE]

PRT Result

Build Number: 5941
Build Status: UNSTABLE
PRT Comment: pytest tests/foreman/api/test_http_proxy.py tests/foreman/cli/test_http_proxy.py --external-logging
Test Result : ===== 1 failed, 9 passed, 2 deselected, 483 warnings in 3291.78s (0:54:51) =====

[Satellite-QE]

PRT Result

Build Number: 5949
Build Status: UNSTABLE
PRT Comment: pytest tests/foreman/api/test_http_proxy.py tests/foreman/cli/test_http_proxy.py --external-logging
Test Result : ===== 1 failed, 9 passed, 2 deselected, 490 warnings in 3247.71s (0:54:07) =====

[Satellite-QE]

PRT Result

Build Number: 5953
Build Status: SUCCESS
PRT Comment: pytest tests/foreman/api/test_http_proxy.py tests/foreman/cli/test_http_proxy.py --external-logging
Test Result : ========= 10 passed, 2 deselected, 466 warnings in 3173.04s (0:52:53) ==========

[vsedmik]

There is some flakiness in the last cli test not reproducible locally.
Since it is not caused by this PR and this PR is just a cherry-pick, we should merge and address the flakiness separately.