find_cluster in vsphere_helper doesn't support host folders

[glennsarti]

It is possible to create host folders so that clusters and single hosts are in a folder hierarchy.

Currently the find_cluster function only looks in the root folder, not sub folders
https://github.com/puppetlabs/vmpooler/blob/master/lib/vmpooler/vsphere_helper.rb#L244-L247

While this is not a common deployment scenario, it is possible that clusters are being missed in this scenario