If the user provides their own ServerProperties bean we want to peek
at it to see if they set the port (and only that) when we are deciding
if the actuator context needs to be created. This happens very early
(in a @Condition) so we need to be very defensive. There are already
quite a few checks in place to prevent a ServerProperties bean from
being instantiated unless we really need it, and yet, when it is
we can do more.

This change creates the bean (and the ManagementProperties) in a
throwaway BeanFactory using the same BeanDefinition as the main
context. This ensures that when the main context bean is created
it will be in the "natural" order and binding to the Environment
can take place as normal.

Fixes gh-4631