Simplify code for checking config availability and SRE deployment status

[craddm]

:white_check_mark: Checklist

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:vertical_traffic_light: Depends on

:arrow_heading_up: Summary

Simplifies the code for checking which configs are available on the SHM and which SREs are deployed. No longer needs to load each stack, and thus is much faster.

:closed_umbrella: Related issues

:microscope: Tests

Tested locally

[github-actions[bot]]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
data_safe_haven/commands
config.py
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