[clustermgtd] Fix spurious errors logged on transient IMDS unavailability

[hehe7318]

Description of changes

This is a follow-up to #704 (commit 1c00989), which added a retry to the compute fleet status retrieval in clustermgtd to "mitigate the impact of networking glitches, such as transient unavailability of IMDS". That retry did not achieve the intended effect, for two reasons:

• its window (3 attempts, 1s apart, so ~2s overall) was shorter than the IMDS outages observed in the fleet, which are in the order of a few seconds, so the retries were exhausted before IMDS became available again;
• every failed attempt was logged as an error by _run_command, so even the attempts recovered by a successful retry left errors in the log, describing failures that no longer had any impact on the cluster.

Either one alone is enough to fail the log assertion, so both are addressed:

• waits are now 1s, 2s and 4s, plus a jitter of up to 1s each to avoid retrying in lockstep with the other daemons polling IMDS. The overall wait is kept well below the clustermgtd loop time, because the fleet status is retrieved synchronously at the beginning of every loop;
• errors on the single attempts are no longer logged, by passing log_error=False. The failure is reported once by get_status, only after the retries are exhausted, so that genuinely prolonged IMDS outages remain visible.

This follows the same principle already established in d8e8b25, where a failure covered by an outer retry was downgraded and the error left to the caller that observes the exhausted retries.

Tests

• Unit test passed
• test_slurm_scaling which this issue was initially detected is ongoing

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