Create Gatekeeper function

[eak13]

With pod security policies being deprecated in k8s v1.21, there is a need for a replacement to apply & manage policies. Gatekeeper provides the means for policy application & enforcement.

Information on PSP deprecation:
https://kubernetes.io/blog/2021/04/06/podsecuritypolicy-deprecation-past-present-and-future/

Information for Gatekeeper can be found here:
https://kubernetes.io/blog/2019/08/06/opa-gatekeeper-policy-and-governance-for-kubernetes/
https://github.com/open-policy-agent/gatekeeper
https://open-policy-agent.github.io/gatekeeper/website/docs/howto/

Tasks for this issue:

• Create a function that deploys gatekeeper
• Define a phase to deliver gatekeeper
• Associate gatekeeper deployment with the multi-tenant type

This will be part of the #86 epic & will provide the initial functionality to deploy gatekeeper. Subsequent issues will be created to define & apply policies.

[lb4368]

PS: https://review.opendev.org/c/airship/treasuremap/+/795601