Rename IMAGE_OPERATOR variable to OPERATOR_IMAGE for clarity

[ptankov]

CHANGE DESCRIPTION

Problem:
Short explanation of the problem.

Cause:
Short explanation of the root cause of the issue if applicable.

Solution:
Short explanation of the solution we are providing with this PR.

CHECKLIST

Jira

• Is the Jira ticket created and referenced properly?
• Does the Jira ticket have the proper statuses for documentation (Needs Doc) and QA (Needs QA)?
• Does the Jira ticket link to the proper milestone (Fix Version field)?

Tests

• Is an E2E test/test case added for the new feature/change?
• Are unit tests added where appropriate?
• Are OpenShift compare files changed for E2E tests (compare/*-oc.yml)?

Config/Logging/Testability

• Are all needed new/changed options added to default YAML files?
• Did we add proper logging messages for operator actions?
• Did we ensure compatibility with the previous version or cluster upgrade process?
• Does the change support oldest and newest supported PXC version?
• Does the change support oldest and newest supported Kubernetes version?