change how the datacenter is found

Instead of using `serviceInstance.find_datacenter` to find the datacenter by name, loop through `rootFolder.children` looking for a child that has that name and is a datacenter.

The reason to do this is that `rootFolder.children` is populated with the children that the user has permissions to see. `serviceInstance.find_datacenter` does `rootFolder.traverse` which requires the permission to search through all the children