feat: implement the driver doctor hook

[tas50]

This driver declares no required_config at all, so nothing catches a missing endpoint or credential at finalize_config! time the way it does in most kitchen drivers. A missing cloudstack_api_key surfaces part way through create, after the run has already started. doctor moves that discovery to kitchen doctor.

Two groups of checks:

Settings the driver cannot deploy without — cloudstack_api_url, cloudstack_api_key, cloudstack_secret_key, cloudstack_template_id, cloudstack_serviceoffering_id, cloudstack_zone_id. Each message says what the setting is for, not just that it is absent:

cloudstack_template_id is not set: CloudStack needs the template to build the instance from.

All of them are reported in one run. Reporting the first and stopping would mean six kitchen doctor invocations to configure a driver from scratch.

The endpoint actually works — the URL parses and has a host, and CloudStack accepts the keys (via a list_zones call, the cheapest authenticated request available).

The connectivity check is skipped when the endpoint or credentials are missing, since the first group already said so and "could not connect" on top of "no API key is set" is noise rather than information.

Verification

• rake test — 69 examples, 0 failures (63 before; 6 new), rake exit 0
• cookstyle --chefstyle — 9 files, no offenses
• yard stats — 100% documented