feat: added env variable to remove managed-by label on argocd instance deletion

[ciiay]

What type of PR is this?
/kind enhancement

What does this PR do / why we need it:
This PR introduces a new environment variable "REMOVE_MANAGED_BY_LABEL_ON_ARGOCD_DELETION", and changes the default behave on namespace with "managed-by" label when the target Argo CD instance gets removed by the gitops-operator. By default, when the Argo CD instance with the target namespace gets deleted, the namespace keeps "managed-by" label value. Only when "REMOVE_MANAGED_BY_LABEL_ON_ARGOCD_DELETION" is set to "true" in the Subscription config, it will remove the "managed-by" label on Argo CD deletion.

Have you updated the necessary documentation?

• Documentation update is required by this PR.
• Documentation has been updated.

Which issue(s) this PR fixes:

Fixes https://issues.redhat.com/browse/GITOPS-1826

How to test changes / Special notes to the reviewer:

Without "REMOVE_MANAGED_BY_LABEL_ON_ARGOCD_DELETION" set to "true" in argocd-operator Subscription

1. Install argocd-operator with namespace A and create an Argo CD instance
2. Create a new namespace B and set "argocd.argoproj.io/managed-by: " label
3. Delete the Argo CD instance with namespace A
4. Check the new namespace B and see if it still has the "managed-by" label which sets to namespace A

With "REMOVE_MANAGED_BY_LABEL_ON_ARGOCD_DELETION" set to "true" in argocd-operator Subscription

1. Same as 1 from above process
2. Edit Subscription of argocd-operator as below

...
spec:
...
  config:
    env:
      - name: REMOVE_MANAGED_BY_LABEL_ON_ARGOCD_DELETION
        value: 'true'
...

3. Same as 2 -3 from above process
4. Check the new namespace B and see the "managed-by" label no longer exists

[wtam2018]

Please update the documentation.

[jannfis]

LGTM. Thanks @ciiay.

[ifaigenb]

In my org I do not have permissions to edit the subscription config but I need this fix as well.
Is it possible to add an option to edit this via the argocd instance object?