allow to produce baremetal BMH resources earlier in the deployment

Today we produce BareMetalHosts resources in the cluster unit:

this does not happen before everything is ready to provision the cluster, in particular not beforce os-image-server has downloaded all OS images, which takes times
sometimes people would like the BMH objects to be created first, to have a chance to look at hw inspection result, before they will finalize the hardware-related settings

My proposition is:

evolve sylva-capi-cluster to introduce a value which will control which resources are created, allowing to have only the BMH created, and allowing all the rest to be created
this would be leveraged to introduce a cluster-bmh unit that will use sylva-capi-cluster with the same values as today the cluster unit, but with this new settings, to let this unit create only the BMH resources
the existing cluster unit would be mostly untouched, it would just receive the values of the new setting that will result in creating all the other resources (everything but BMHs)

Thanks to the above:

we can have BMH be created much earlier during bootstrap, allowing the hardware inspection to happen in parallel with the OS image download
people will be able to disable the cluster unit during to have inspection alone, by temporarily setting this in values:

units:
  cluster:
    enabled: false