[#70162466] Fog leakage

While updating vCloud Launcher I realised that `Vcloud::Core::Vapp` had a method called `fog_vms`, which returns the VMs for that vApp. There's no reason for this to be called `fog_vms` and it's another way it leaks stuff about fog to the calling apps.

I've renamed it to `vms`. Potentially it could be `vcloud_vms` to indicate something about it coming from the API, but that would commit us to this implementation, about which we already have a number of issues, e.g. https://www.pivotaltracker.com/story/show/68787298.