[GB200] Restore locking mechanism to update IMEX nodes config.

[gmarciani]

Description of changes

Restore locking mechanism to update IMEX nodes config.
We removed it in aws/aws-parallelcluster-cookbook#3029 as we considered it overkilling after moving to local config files. However, having the locking mechanism prevents impacts on IMEX when concurrent jobs are submitted.

Tests

OGNOING test_gb200

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[gmarciani]

Cancel this PR as we just decided to enforce exclusive job execution by exiting immediately from the prolog if job is not exclusive. In that scenario we do not need any file locking

[gmarciani]

Closed in favor of #7019