invalid config can allow updateConfig to put controller in invalid state

[alexec]

Summary

See #2066, it is possible for updateConfig to fail at various points leaving the controller in a potentially invalid or unrecoverable state.

Motivation

Avoid having the controller in a bad state.

Proposal

Rather than load the config, and then iterate over values returning an error part way thought, we should create and validate a replacement configuration and set it all at once.

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Message from the maintainers:

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[alexec]

Additionally, we ignore deletion for the cm.