Merge pull request #13 from hrabbouh/master

New fix for afterDisconnect running twice, and builds attempting to start on a disconnecting slave.
Added graceful shutdown to the Power Down build step
Merge branch 'master' of https://github.com/jenkinsci/vsphere-cloud-plugin
[maven-release-plugin] prepare release vsphere-cloud-1.1.11
[maven-release-plugin] prepare for next development iteration