Allow the number of VMs up at any one time to be limited.

Code adapted from the Jenkins vSphere cloud plugin at commit "f3548bffde" (https://github.com/jenkinsci/vsphere-cloud-plugin/tree/f3548bffde1d268270464236f59fce68474de2f5).
Merge branch 'master' of
git://github.com/bryandagnin/libvirt-slave-plugin into
bryandagnin-master

(Implementing JENKINS-17293)