[FIXED JENKINS-21999] If a slave node does not exist it will throw a
null pointer exception. Instead we create a dummy launcher and return
that if it is unable to get the slave node.
Throw IOException if trying to launch a DummyLauncher. Restrict
DummyLauncher class to no external use.
Merge branch 'JENKINS-21999' of github.com:christ66/jenkins
[JENKINS-21999] Noting merge of #1145.