Allow zpool configuration to be undefined for specific nodes

This does not impact existing nodes with the configuration already set (be it manually or through puppet [1]).

This will allow to configure storage1.staging as a rancher agent.

[1]

swh-octo-diff rancher-metal-02

*** Running octocatalog-diff on host rancher-node-metal02.internal.softwareheritage.org
I, [2023-11-23T13:47:30.005214 #1297165]  INFO -- : Catalogs compiled for rancher-node-metal02.internal.softwareheritage.org
I, [2023-11-23T13:47:30.255913 #1297165]  INFO -- : Diffs computed for rancher-node-metal02.internal.softwareheritage.org
I, [2023-11-23T13:47:30.255951 #1297165]  INFO -- : No differences
*** End octocatalog-diff on rancher-node-metal02.internal.softwareheritage.org
+ set +x
/home/tony/work/inria/repo/swh/puppet-environment/swh-site

[2]

swh-octo-diff storage1.staging

diff origin/production/storage1.internal.staging.swh.network current/storage1.internal.staging.swh.network
*******************************************
  Zpool[data] =>
   parameters =>
     disk =>
      - vdb
*******************************************
*** End octocatalog-diff on storage1.internal.staging.swh.network
+ set +x
/home/tony/work/inria/repo/swh/puppet-environment/swh-site

This does work in the impacted node (whereas it failed previously).

root@storage1:~# puppet agent --enable; puppet agent --test --noop; puppet agent --disable
Info: Using configured environment 'staging'
Info: Retrieving pluginfacts
Info: Retrieving plugin
Info: Retrieving locales
Info: Loading facts
Info: Applying configuration version '1700743750'
Notice: /Stage[main]/Profile::Zfs::Kubelet/Zfs[data/kubelet]/ensure: current_value 'absent', should be 'present' (noop)
Notice: Class[Profile::Zfs::Kubelet]: Would have triggered 'refresh' from 1 event
Notice: /Stage[main]/Profile::Zfs::Rancher/Zfs[data/rancher]/ensure: current_value 'absent', should be 'present' (noop)
Notice: /Stage[main]/Profile::Zfs::Rancher/Zfs[data/volumes]/ensure: current_value 'absent', should be 'present' (noop)
Notice: Class[Profile::Zfs::Rancher]: Would have triggered 'refresh' from 2 events
Notice: Stage[main]: Would have triggered 'refresh' from 2 events
Notice: Applied catalog in 13.76 seconds

Refs. swh/infra/sysadm-environment#5164 (closed)