As the cloud relies more on container env vars than system properties, we should allow for transparent resolution of expressions from env vars as well as the current system property mechanism.

For example, if I have the following xml

<element some-attribute="${test.my-property:default}"/>

At present you need to start the server with -Dtest.my-property=hello (or via a model entry for the property) to replace the value at runtime.

This RFE proposes that if the env var TEST_MY_PROPERTY=hola is set when starting the server, we use the value of the environment variable.

To keep this similar to how MP Config does the same thing, the system property will take precedence over the environment variable if defined.