Reintroduce "ha" option refactoring (empty it, and add infra-capd)

This MR:

• reintroduces !452 (merged) after the revert in !459 (merged)

• fixes the error reported in sylva-projects/sylva-core!8380 (merged) by having ha.yaml include a dummy value _internal.ci_is_ha instead of {}

• fixes another error: execution error at (sylva-units/templates/units.yaml:6:31): requested control_plane_replicas=3: can't continue, first install happened in non-HA mode, this value should be 1

  • if cluster.control_plane_replicas is a gotpl snippet rather than an int (control_plane_replicas: '{{ ternary 1 3 (eq .Values.cluster.capi_providers.infra_provider "capd") | include "preserve-type" }}'), because the computation done here for _internal.ha_cluster.checkpoint did not ensure that cluster.control_plane_replicas was interpreted before being used, it leads to bogus computation (int applied to our gotpl snippet string gives 0).

Testing in sylva-projects/sylva-core!8381 (closed) which reapplies the same changes from sylva-projects/sylva-core!8365 (merged) using this branch.

LE. by @tmmorin: most recent test was done in sylva-projects/sylva-core!8398 (merged)