[6.15.z] variable value should del after update

[Satellite-QE]

Cherrypick of PR: #16969

I extended the existing test case to include the following steps:

1. Create an Ansible variable with a default value and an overridden key.
2. Update the value of the Ansible variable at the host level.
3. After updating, delete the value, ensuring that it reverts to the default value.

Dependent PR: SatelliteQE/airgun#1643

[Satellite-QE]

trigger: test-robottelo
pytest: tests/foreman/ui/test_ansible.py::TestAnsibleCfgMgmt -k test_positive_assign_ansible_role_variable_on_host
airgun: 1643

[Satellite-QE]

PRT Result

Build Number: 9568
Build Status: UNSTABLE
PRT Comment: pytest tests/foreman/ui/test_ansible.py::TestAnsibleCfgMgmt -k test_positive_assign_ansible_role_variable_on_host --external-logging
Test Result : ========== 1 failed, 10 deselected, 26 warnings in 867.61s (0:14:27) ===========

[lhellebr]

@amolpati30

>           assert new_value not in reset_variable_value
E           assert '["test update"]' not in '["test update"]'
E             
E             '["test update"]' is contained here:
E               ["test update"]

tests/foreman/ui/test_ansible.py:260: AssertionError