Fix LSF ClusterEnvironment host limitation

[awaelchli]

What does this PR do?

There is a funny issue with the LSF cluster controller. If you submit a job with too many nodes, it will stop setting the environment variable LSB_HOSTS which our cluster environment plugin relies on. The docs mention this here:

For parallel jobs, several CPUs are used, and the length of LSB_HOSTS can become very long. sbatchd needs to spend a lot of time parsing the string. If the size of LSB_HOSTS exceeds 4096 bytes, LSB_HOSTS is ignored, and sbatchd sets only LSB_MCPU_HOSTS.

So the fix we are proposing is to only rely on LSB_MCPU_HOSTS and cross fingers.

Fixes #10216 (to be verified by OP)

Original PR that added LSF plugin: #5102

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Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

[tchaton]

Looks great ! Really neat !