JcloudsLocation: No NPE if setup.script is given but not setup.script.vars
Correct typo in Reference and remove duplicate pom dependency in test-support.
Adds MACHINE_CREATE_ATTEMPTS JcloudsLocation config

Use to retry VM creation when exceptions are thrown, e.g. when AWS VMs
are dead on arrival. (Has successfully kept at least one run of
brooklyn-marklogic demo on track.)
Retry VM creation: Addressed review comments, added unit test.
Merge pull request #964 from sjcorbett/retry-vm-create

Retry vm create