Review Request: 8006658: build-infra: Make MILESTONE behave the same as JDK_BUILD_NUMBER

[Erik Joelsson]

In JDK-8001895, MILESTONE and JDK_BUILD_NUMBER were made customizable. 
But by mistake, JDK_BUILD_NUMBER could get picked up from the 
environment while MILESTONE couldn't. They should behave the same and at 
this time, listening to the environment is needed.

http://cr.openjdk.java.net/~erikj/8006658/webrev.root.01/
/Erik