Simplify cloud access profile setup

[ideaship]

Rewrite step 3 ("Configure your cloud access profile") of the testbed deployment guide to provide a single fast path using application credentials downloaded from Horizon.

The old version had several problems (see issue #960): clouds-public.yaml was mentioned without context, there was no guidance on application credential settings, no fast path for cloud access configuration, and it was unclear what credentials a provider actually supplies. The section tried to cover different cases (app credentials vs username/password, secure.yaml, profiles, cloud naming constraints) without offering a simple straight path.

Investigation showed that the downloaded clouds.yaml from Horizon already contains auth_url, region_name, and identity_api_version — all the information that
clouds-public.yaml provides via the profile parameter. The only extra field in clouds-public.yaml is image_format (for 4 providers), but Terraform does not use it. Default app credential settings (empty roles, Unrestricted=false) are sufficient, confirmed by a successful deployment.

The rewrite removes the username/password alternative (no clouds.yaml download available from Horizon, requiring manual YAML construction), the clouds-public.yaml/profile explanation, and the secure.yaml option. What remains is a linear set of concrete steps: create app credential in Horizon, download clouds.yaml, rename the cloud entry, and copy the file.

Closes: #960

AI-assisted: Claude Code

[github-actions]

❌MegaLinter analysis: Error

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
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✅ JSON	jsonlint	4		0	0	0.13s
✅ JSON	prettier	4		0	0	0.39s
✅ JSON	v8r	4		0	0	5.73s
✅ MARKDOWN	markdownlint	156		0	0	2.33s
✅ MARKDOWN	markdown-table-formatter	156		0	0	0.41s
✅ REPOSITORY	checkov	yes		no	no	17.71s
✅ REPOSITORY	git_diff	yes		no	no	0.04s
✅ REPOSITORY	secretlint	yes		no	no	1.97s
✅ REPOSITORY	trufflehog	yes		no	no	4.38s
✅ SPELL	codespell	164		0	0	0.47s
❌ SPELL	lychee	164		1	0	20.1s
✅ YAML	prettier	4		0	0	0.35s
✅ YAML	v8r	4		0	0	5.66s
✅ YAML	yamllint	4		0	0	0.51s

Detailed Issues

❌ SPELL / lychee - 1 error

[TIMEOUT] https://bugs.launchpad.net/keystonemiddleware/+bug/2129018 | Timeout
📝 Summary
---------------------
🔍 Total..........790
✅ Successful.....732
⏳ Timeouts.........1
🔀 Redirected.......0
👻 Excluded........57
❓ Unknown..........0
🚫 Errors...........0

Errors in docs/appendix/security/ossa-2026-001.md
[TIMEOUT] https://bugs.launchpad.net/keystonemiddleware/+bug/2129018 | Timeout

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