CASM-4855 Make image signature validation rules customizable

[mtupitsyn]

Summary and Scope

Image signature validation rules must be customizable. Customers may have their own keys (in addition to our keys) or even validation configs. Standard way of tuning deployments with customer specific parameters is using customizations.yaml file. It is used during fresh installs. During upgrades, existing configuration is preserved via k8s secret, and gets amended via update-customizations.sh script - this way we may alter configuration during future upgrades (for example, remove outdated keys).

This change also fixes misconfiguration which causes CASMTRIAGE-7287.

All PR's in this batch:

• https://github.com/Cray-HPE/shasta-cfg/pull/18
• CASM-4855 Make image signature validation rules customizable csm#3648
• CASM-4855 Make image signature validation rules customizable #14
• CASM-4855 Make image signature validation rules customizable docs-csm#5372

Issues and Related PRs

• Resolves CASM-4855
• Resolves CASMTRIAGE-7287

Testing

Tested on:

• Virtual Shasta

Test description:

Deployed temporary build, ensured that check image policy is configured correctly.

Risks and Mitigations

Low - we change the way things configured, but resulting configuration remains the same.