Prevent associating multiple ElementalHosts

[anmazzotti]

Props to @juadk for finding this corner case.

For context, the ElementalMachine <--> ElementalHost association works like the following, from the ElementalMachineController perspective:

1. Patch the ElementalHost candidate to reference the ElementalMachine (ElementalHost --> ElementalMachine)
2. Patch the ElementalMachine to reference the ElementalHost candidate (ElementalMachine --> ElementalHost)

It can happen that 2. fails after 1. succeeds, this will lead to another ElementalMachine reconcile loop, where we are going to find a new ElementalHost. The one already linked is going to be excluded from a followup search, since it's already labeled as associated.

It will look like this, where the same ElementalMachine is associated to 2 (or more) ElementalHosts:

gh-runner@elemental-e2e-ci-379bc19f05ef448dae0f2808ee53cec9:~> kubectl get elementalhosts -A
NAMESPACE   NAME       CLUSTER             MACHINE                                 ELEMENTALMACHINE                        PHASE     READY   AGE
default     node-001   elemental-cluster   elemental-cluster-control-plane-df69z   elemental-cluster-control-plane-x7g5d   Running   True    8m14s
default     node-003   elemental-cluster   elemental-cluster-md-0-tdtqm-pzpk6      elemental-cluster-md-0-tdtqm-pzpk6      Running   True    8m14s
default     node-002   elemental-cluster   elemental-cluster-md-0-tdtqm-pzpk6      elemental-cluster-md-0-tdtqm-pzpk6      Running   True    8m14s

This patch introduces a prior lookup to find any already-linked ElementalHost, and use it as a candidate to finalize association.

[anmazzotti]

Just a random change.
This makes the tests now take ~26seconds in total, from ~24 (on my system), however the logs are a thousand times less spammy when things fail.

[fgiudici]

looks good!