[6.13.z] Remove nailgun.entities imports in tests/foreman/endtoend

[Satellite-QE]

Cherrypick of PR: #15083

Problem Statement

SAT-22502

Uses of nailgun.entities.${ENTITY} should be replaced with ${SATELLITE_INSTANCE}.api.${ENTITY}.

Solution

All tests under tests/foreman/endtoend/ now use nailgun through the satellite instance's api attribute, and imports of nailgun.entities have been removed.

[Satellite-QE]

trigger: test-robottelo
pytest: tests/foreman/endtoend/test_api_endtoend.py -k 'test_positive_find_default_org or test_positive_find_default_loc or test_positive_find_admin_user'

[Satellite-QE]

PRT Result

Build Number: 6966
Build Status: SUCCESS
PRT Comment: pytest tests/foreman/endtoend/test_api_endtoend.py -k test_positive_find_default_org or test_positive_find_default_loc or test_positive_find_admin_user --external-logging
Test Result : ========== 3 passed, 3 deselected, 35 warnings in 1098.69s (0:18:18) ===========