Allow link installation to VM Cluster

[ilimit-devops]

Allow to link installations on "VM Cluster" objects, like "Kubernetes Cluster".

Deployments or StateFulSets are not VM, but reside in a "VM Cluster" (Kubernetes)

[MartVisser]

Looks like a good idea to me.

A third type of link to 'Installations'. So adding 'Virtualization Clusters' (/virtualization/clusters/) in addition to 'Virtual Machines' and 'Devices'

[wkoot]

Currently we have the property SoftwareProductInstallation.platform which is either a device virtualmachine.
The fact that the option is binary, is secretly used in the Installations table ordering for type and platform.
Things work by simply adding cluster, but perhaps we should now revisit this logic as it is counter-intuitive.

Also, we currently allow linking both devices and virtualmachines but the code always favours device if present.
I have added a constraint to the model that only one of the three (device, VM, or cluster) is allowed to be present.
Docs say: A cluster of VirtualMachines. Each Cluster may optionally be associated with one or more Devices.
This should therefore not be a problem, functionally speaking. But this might break if someone has incompatible data.

Should we support installations which are not linked to a platform? Does it make sense to have them?