Remove mandatory field from OCIClusterSpec and use webhook validation instead

[joekr]

What would you like to be added:
We want to remove the mandatory from CompartmentId string `mandatory:"true" json:"compartmentId"` in the OCIClusterSpec and setup the mandatory validation in ocicluster_webhook

Why is this needed:
This seems to be the preferred way to do validation and it will allow use to remove the compartmentId: REPLACE defined in templates/clusterclass-example.yaml