Add vco provisioning

(gh-590) Add vCenter Orchestrator (vCO) support to Beaker

Without this patch beaker cannot use vCO to provision/cleanup virtual machines, which allows for the workflow that creates/destroys a vm to be customised to include various other steps that beaker will likely never support such as adding DNS, bootstrapping the guest OS, setting up password-less ssh login and creating appropriate hiera data.

This patch uses rest-client to issue REST API calls to a vCO host to execute a "provisioning" workflow and "cleanup" workflow where required. It uses the existing fog file to store vco credentials and allows for the workflow name and parameters to be set in the nodeset file.