HTTP Proxy customer cases update

[vsedmik]

This PR just addresses a few changes from the component eval

• adds a case for insights client registration via HTTP proxy
• removes old stub covered in foreman-maintain
• updates http_proxy fixture to remove the proxy on teardown

PRT test Cases example

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

[vsedmik]

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

[Satellite-QE]

PRT Result

Build Number: 6186
Build Status: SUCCESS
PRT Comment: pytest tests/foreman/cli/test_http_proxy.py -k insights --external-logging
Test Result : ========== 6 passed, 3 deselected, 428 warnings in 3326.86s (0:55:26) ==========

[vsedmik]

rebased

[vsedmik]

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

[Satellite-QE]

PRT Result

Build Number: 6779
Build Status: UNSTABLE
PRT Comment: pytest tests/foreman/cli/test_http_proxy.py -k insights --external-logging
Test Result : ===== 3 failed, 3 passed, 3 deselected, 479 warnings in 2910.62s (0:48:30) =====

[Satellite-QE]

PRT Result

Build Number: 6793
Build Status: SUCCESS
PRT Comment: pytest tests/foreman/cli/test_http_proxy.py -k insights --external-logging
Test Result : ========== 6 passed, 3 deselected, 495 warnings in 2965.79s (0:49:25) ==========