fix(keycloak): Add startup probe to catch slow startup

When using infinispan the app startup is a bit slower. As a result,
it'll might cause a termination of the keycloak instance before it's
fully in sync and therefore breaks HA. This patch introduces a startup
probe that resolves this issue by waiting for a successful startup
before the livenessprobe kicks in and might terminates the pod at any
point when it's actually failing again.

References:
https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#define-startup-probes