Allow number of VMs online to be limited

Adds an option to the Hypervisor configuration that allows the number of VMs online at once to be limited.

Code adapted from the Jenkins vSphere cloud plugin at commit "f3548bffde" (https://github.com/jenkinsci/vsphere-cloud-plugin/tree/f3548bffde1d268270464236f59fce68474de2f5).