system tests for VMware vCenter functionality

- test basic OpenStack with vCenter functionality
- when vCenter is used as hypervisor, Cinder must be configured to use
  VMDK backend (`volumes_vmdk' must be True), otherwise OSTF tests will
  fail
- implement following scenarios:
   vcenter_multiple_cluster
     deploy cluster with one controller that uses vCenter with
     two vSphere clusters, verify that we have two hypevisors
   vcenter_ha
     deploy cluster with 3 controllers and vCenter as hypervisor
   vcenter_simple_add_cinder
     deploy cluster with one controller, add standalone cinder node
   vcenter_ha_deployment_with_cinder
     deploy HA cluster with standalone cinder node, run checks
   vcenter_simple_stop_deployment
     deploy cluster with one controller, stop deployment process, start
     it againt, run checks

Closes-bug: #1368285
Change-Id: I3dddd6fb80d3acd5ef1a03de2004ec1e5da4c85d
Merge "system tests for VMware vCenter functionality"