capm3: move BareMetalHosts into new 'cluster-bmh' unit

Closes #980 (closed)

• This MR makes use of sylva-projects/sylva-elements/helm-charts/sylva-capi-cluster!275 (merged) which evolves sylva-capi-cluster to introduce a new resource_groups value which will control which resources are created, allowing to have only the BMH created
• this is leveraged in this MR to introduce the new cluster-bmh unit that uses sylva-capi-cluster with the same values as the ones for the cluster unit, but with this new resource_groups: [baremetal-hosts] settings to have this unit create only the BMH resources
• the existing cluster unit is mostly untouched, it just receives the new resource_groups to generate all the other resources (everything but BMHs)
• some changes are made to the boostrap code so that the cluster-bmh is disabled/re-enabled at the same time as the cluster unit

Result:

• the BMHs are created much earlier in the deployment (in particular in parallel with the os-image-server image download)

• the deployment can now be done in two stages: inspection and real deployment

  • 1. set values with the cluster unit disabled

  units:
    cluster:
      enabled: false

  • 2. run bootstrap.sh
  • 3. look at BMHs status to see inspection data and adjust deployment values
  • 4. remove the lines that disable the cluster unit
  • 5. run bootstrap.sh again