Integrate OPA / Gatekeeper security

[etavela]

• [x] Install and configure Gatekeeper for cluster
• [x] Install basic rule(s)
  • [x] Restrict image repositories
  • [x] Check after building helloworld and test stage
  • [x] Examine other rules to apply
• [x] Check in DigitalOcean (requires cert-manager, k8ssandra, etc.)
• [x] After whitelist is tested, restrict use of images from non-whitelisted sources
  • [x] Verify that deployment is blocked
• [x] Create a ticket to configure Gatekeeper rules later