XD-1996 Tightening null checks
XD-1509 Disable publishing hadoop artifacts

- Disable publishing hadoop distro artifacts
  - Check for the existence of `artifactory-publish` plugin
before disabling `artifactoryPublish`
XD-1949b - Ensure DSM matrix is diagonal

* Created new exception, the somewhat odd named
  BatchJobAlreadyExistsInRegistryException, that replaces the
  BatchJobAlreadyExistsException that references absence in the JobLocator.
* This exception block can likely be since this BBP should only be processing
  jobs that were not deployed.  Created https://jira.spring.io/browse/XD-2014
  to track.
XD-1509 Disable publishing modules artifacts

- Disable modules projects' artifact publishing by disabling
`artifactoryPublish` task if defined for the module project
- Use the same approach for hadoop artifacts
XD-1335 Fix STS Gradle Import

Gradle plugin import fails due to missing directory for
spring-xd-exec.

Add a README to force the directory to exist.