When loading var_container_storage / var_template_storage from default.vars
or app defaults, validate that the storage actually exists and is active on
the current node. In mixed clusters (e.g. LVM-Thin + ZFS hosts), saved
defaults from one node type would fail on another because the referenced
storage doesn't exist. Now invalid storage values are skipped with a
warning, allowing the normal storage selection prompt to appear.
Closes#12766