When listing virtual machines under a compute_resource on VMware vSphere a folder hash is built of all folders under the datacenter, but the datacenter's vmFolder is skipped in the PropertyFilterSpec.

This causes VMs which have the vmFolder as their parent to throw an undefined method '[]' for nil:NilClass error when they try to access 'folder_inventory[vm['parent']._ref][:path]'.

Traceback:
2018-03-06T11:11:34 fd0ae69d [app] [W] Error has occurred while listing VMs on dev-vc65 (VMware) | NoMethodError: undefined method `[]' for nil:NilClass | /home/agrare/src/foreman/app/services/fog_extensions/vsphere/mini_servers.rb:61:in `lookup_parent_folders' | /home/agrare/src/foreman/app/services/fog_extensions/vsphere/mini_servers.rb:62:in `lookup_parent_folders' | /home/agrare/src/foreman/app/services/fog_extensions/vsphere/mini_servers.rb:56:in `block in set_folder_paths' | /home/agrare/src/foreman/app/services/fog_extensions/vsphere/mini_servers.rb:55:in `each' | /home/agrare/src/foreman/app/services/fog_extensions/vsphere/mini_servers.rb:55:in `set_folder_paths' | /home/agrare/src/foreman/app/services/fog_extensions/vsphere/mini_servers.rb:50:in `generate_folder_inventory' | /home/agrare/src/foreman/app/services/fog_extensions/vsphere/mini_servers.rb:21:in `all'