Description of problem:

User with access to the namespace can approve any task on behalf of any user

Prerequisites (if any, like setup, operators/versions):

Pipelines 1.15.0 (index image 739353)
OpenShift 4.16

Steps to Reproduce

1. Create a pipeline with approval task in a new namespace
2. give "user1" permissions to access this namespace
3. run the pipeline
4. list approval tasks - opc approvaltask list
5. try to approve task using CLI - opc approvaltask approve <task_name>
6. try to approve task by editing it - oc edit approvaltask <task_name>

Actual results:

correct behavior:

$ opc approvaltask approve manual-approval-pipeline-eeuwp1-task-2
Error: failed to approve approvalTask from namespace manualtest: Approver: user1, is not present in the approvers list

wrong behavior:

oc edit approvaltask manual-approval-pipeline-eeuwp1-task-2

# set "approve" for user "foo" which actually doesn't exist at all

approvaltask.openshift-pipelines.org/manual-approval-pipeline-eeuwp1-task-2 edited

Expected results:

User's input is validated

Reproducibility (Always/Intermittent/Only Once):

Always

Acceptance criteria: 

Definition of Done:

Build Details:

Additional info (Such as Logs, Screenshots, etc):

 *