Improve UMA enforcer configuration when using Spring Boot

As it stands today, developers using Spring Boot must create a specific bean in order to be able to enabla UMA to the policy enforcer.

The reason for this is that Spring Boot does not handles empty configuration such as policy-enforcer.user-managed-access= thus requiring at least one property to be set. Or creating a specific bean to set this setting.

We should improve usability in a way that developers can enable UMA more easily.