Add Namespace support to UseServiceVersion
Add Namespace support to UseServiceVersion

The UseServiceVersion method was hardcoded to the vim25 namespace,
allow the method to be used with other namespaces.  For example:

  c := cns.NewClient(...)
  err = c.UseServiceVersion("vsan")

vmware/govmomi,vmware/govmomi,bruceadowns/govmomi,vmware/govmomi,bruceadowns/govmomi,bruceadowns/govmomi