Multi container testing module

Introduction of Spring XD multi container tests under the module:

  spring-xd-distributed-test

External JVM launch support provided by the Oracle Tools framework:

  https://java.net/projects/oracletools

Since these tests have the potential to add multiple minutes to the
build, they are not enabled by default. Include the following system
property command to include distributed test execution:

  -Drun_distributed_tests=true