Reconsider expressions for model reference attributes in server-group and server-config

There are some attributes in the server-group and server-config resources that represent references to other resources that support expressions. For example the "group" attribute in a server-config and the "profile" attribute in a server-group. Supporting expressions on such attributes is problematic because it precludes accurately validating that the reference is valid during operation execution Stage.MODEL. This is because expression resolution sources may not be fully populated until Stage.RUNTIME.

I wanted to remove expression support from these attributes but was unwilling to do so because of concerns about breaking compatibility. But some recent testing showed me that on master for at least some of these, setting an expression would not work – the HC could not launch servers. If these expressions didn't work in previous releases as well, we can remove support for them without breaking compatibility.

The task here is to test setting these attributes to expressions in 7.1.2 and 7.1.3. For any attribute like this where expression support didn't actually work, we should remove the support in 7.2.

Testing can be done by simply taking the current xml attribute value, e.g. "foo" and using it as the default in an expression, e.g. "${exp-test:foo}" and then starting the HC. There is no need to mess with setting system property "exp-test".