feat(linux): refactor secure-tls-bootstrap.service to use default file and conditionally set AZURE_ENVIRONMENT_FILEPATH

[cameronmeissner]

What this PR does / why we need it:

refactors secure TLS bootstrapping unit to use a default file, similar to kubelet, for environment variable configuration - this allows us to easily specify the AZURE_ENVIRONMENT_FILEPATH environment variable when needed (stack clouds), such that the secure TLS bootstrapping client can correctly resolve in-cloud Entra ID endpoints for service principal token acquisition

Which issue(s) this PR fixes:

Fixes #

[Copilot]

Pull request overview

This PR refactors the Linux secure TLS bootstrapping systemd drop-in generation to use a dedicated default environment file (mirroring kubelet’s /etc/default pattern), and conditionally injects AZURE_ENVIRONMENT_FILEPATH when it is set to support stack/custom cloud endpoint resolution.

Changes:

• Write /etc/default/secure-tls-bootstrap containing BOOTSTRAP_FLAGS=..., and optionally AZURE_ENVIRONMENT_FILEPATH=....
• Update the secure-tls-bootstrap systemd drop-in to reference the new file via EnvironmentFile=... instead of embedding Environment="BOOTSTRAP_FLAGS=...".
• Extend ShellSpec coverage to validate both the default-file creation and the conditional AZURE_ENVIRONMENT_FILEPATH behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
parts/linux/cloud-init/artifacts/cse_config.sh	Creates /etc/default/secure-tls-bootstrap and switches the secure TLS bootstrapping drop-in to EnvironmentFile=/etc/default/secure-tls-bootstrap.
spec/parts/linux/cloud-init/artifacts/cse_config_spec.sh	Updates/expands unit tests to assert the new default file contents and conditional AZURE_ENVIRONMENT_FILEPATH inclusion.