Cap k3d create_cluster retries at three attempts

[SebTardif]

Stop unbounded retries in k3d.create_cluster.

Problem

K3D.create_cluster retries forever when k3d cluster create keeps failing:

p = subprocess.run(cmd)
while p.returncode != 0:
    logger.error('Failed to create k3d cluster, retrying')
    self.delete_cluster(name)
    time.sleep(5)
    p = subprocess.run(cmd)

Kind already aborts after a few create failures. #419 caps delete_cluster. If delete succeeds and create still fails (bad image, docker error), this loop never exits.

Change

Raise RuntimeError("Failed to create k3d cluster") after three create attempts (two delete-and-retry cycles). No other k3d formatting.

Validation

pytest acto/kubernetes_engine/test_k3d_create.py

• Red (unfixed): the raise test hung (5s timeout) because the loop never ended.
• Green: 2 passed (three failures raise; success on the second create).

Origin

The loop has been in 153ef757 / #155 (2022-08-05, Yuxuan).

Ref #413
Ref #419