Set priorityClassName for Gatekeeper instance

1. Proposed title of this feature request

--> Add "priorityClassName" feature for Gatekeeper instances.

2. What is the nature and description of the request?

--> All the possible values that can be configured for gatekeeper resource are documented in [1], however the customer want to include "priorityClassName" within the operator.

3. Why does the customer need this? (List the business requirements here)

--> Customer need this to ensure that their infrastructure components (gatekeeper is one of those) are running with the highest priority.

4. List any affected packages or components.

--> gatekeeper pods

[1] https://access.redhat.com/articles/6047221