[EmbeddedAnsible] Fix manager ref for credentials

[NickLaMuro]

Makes sure we are properly assigning the manager (resource) when creating AutomationManager::Credential records.

Also updates the specs, not only to test this on create, but to also properly initialize the records for the UPDATE and DELETE specs (how I instantiated things were should have been a bit of a tip off that this was coded wrong... oh well...)

Steps for Testing/QA

TODO: Want to confirm this fixes a few things on a VM, but need to make a separate branch to integrate this commit with the work in #18969 to confirm this is doing what I want it to.

• Enable EmbeddedAnsible
• Add a credential
• Create a catalog and catalog item
• When ordering the catalog item, you should see the credential in the dropdown:

[NickLaMuro]

cc @carbonin

This should fix the issue we were discussing earlier today.

@miq-bot add_labels embedded ansible, bug

[miq-bot]

Checked commit NickLaMuro@5d7ebae with ruby 2.4.6, rubocop 0.69.0, haml-lint 0.20.0, and yamllint 1.10.0
2 files checked, 1 offense detected

spec/models/manageiq/providers/embedded_ansible/automation_manager/credential_spec.rb

• ❗ - Line 49, Col 9 - Performance/RedundantMerge - Use params_to_attributes[:resource] = manager instead of params_to_attributes.merge!(:resource => manager).