Align ManagementServerPortUtils strategy from boot 2

[spencergibb]

ManagementServerPortUtils.java was ported from boot (as it was not public). It is likely old and not up to date.

It is used in two places:
https://github.com/spring-cloud/spring-cloud-commons/blob/master/spring-cloud-commons/src/main/java/org/springframework/cloud/client/serviceregistry/AbstractAutoServiceRegistration.java#L156

and

https://github.com/spring-cloud/spring-cloud-commons/blob/master/spring-cloud-commons/src/main/java/org/springframework/cloud/client/serviceregistry/AbstractAutoServiceRegistration.java#L195

See new boot strategy https://github.com/spring-projects/spring-boot/blob/master/spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/web/server/ManagementPortType.java

[ryanjbaxter]

@spencergibb could you add some details to the description of the issue? If you think this can be done in Hoxton.SR1 can you move it there?