Clarify use of the testsuite/integration/elytron/enable-embedded-messaging.cli file

The combo of WFLY-17918 and WFLY-18017 seems to make the testsuite/integration/elytron/enable-embedded-messaging.cli obsolete. Before this file is removed though it should be analyzed to ensure it can be removed.

While not a good place to store the file, this is actually a good example of adding an embedded messaging server with Elytron. Keeping does not seem unreasonable to me as well even though it's not used. Or it could be migrated to documentation.