Make the SONiC port_config path configurable

[ideaship]

First of the series tracked in #2562, which explains the ordering and what each
PR covers. This one stands on its own — it is a production change with an
unchanged default, and nothing later in the series is needed to review it.

The problem

PORT_CONFIG_PATH was hardcoded to /etc/sonic/port_config, which exists only
inside the conductor image: Containerfile:21 copies files/sonic/port_config/
there. A generator run from a checkout instead — local development — finds
nothing at that path, and the workaround is to plant the .ini files under
/etc/sonic, which needs root.

The change

Add SONIC_PORT_CONFIG_PATH to osism/settings.py, following the existing
SONIC_* environment variable convention, and wire
constants.PORT_CONFIG_PATH to it.

The default is the previous hardcoded path, so container behaviour is
identical and nothing in production moves. Setting the variable points the
generator at files/sonic/port_config/ in the checkout instead.

Tests

Three cases: the setting's default, the environment override, and the
settings-to-constants wiring — the last because constants.PORT_CONFIG_PATH is
assigned at import time, so a test that only checked the setting would not catch
the wiring being dropped.

[sourcery-ai]

Hey - I've left some high level feedback:

• In test_port_config_path_follows_settings, consider using monkeypatch.setattr (and possibly the existing reload_settings fixture) instead of manually mutating settings_module.SONIC_PORT_CONFIG_PATH and handling reloads with try/finally, to keep test setup/teardown more consistent and less error-prone.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In `test_port_config_path_follows_settings`, consider using `monkeypatch.setattr` (and possibly the existing `reload_settings` fixture) instead of manually mutating `settings_module.SONIC_PORT_CONFIG_PATH` and handling reloads with `try/finally`, to keep test setup/teardown more consistent and less error-prone.

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