Server runtimes should be able to customize execution-environment

Servers currently cannot customize their execution environment. This should be changed.

UseCase:

1) User has workspace with 1 java 7 installation and 2 java 8 installations
2) User creates a wildfly 8.1.x, which has minimum exec-env of java7
3) User wants to set the runtime to run with execution-environment of java8
4) User sets default jre for java8 to be java8-installation1
5) User starts server, verifies java8 installation1 is used
6) user stops server
7) User changes default jre for java8 execution environment to java8 - installation 2 in the execution-environments preference page
8) User starts wildfly, expects java8 installation2 to be used.