feat: implement the driver doctor hook

[tas50]

kitchen doctor had nothing to say for this driver. Two groups of checks, both about failures that currently arrive part way through create.

Credentials. openstack_username, openstack_api_key, and openstack_auth_url are in required_server_settings, but that only means Fog is handed them even when nil — nothing rejects a nil before the request goes out, so a missing password becomes a Keystone rejection mid-create. Each message names the setting and every place it can legitimately come from, since this driver reads three:

openstack_username is not set: set it in kitchen.yml, export OS_USERNAME, or put it in clouds.yaml.

When all three are present, they are actually exercised against Keystone.

Image and flavor selection. create raises on these:

raise(ActionFailed, "Cannot specify both image_ref and image_id") if config[:image_id] && config[:image_ref]
raise(ActionFailed, "Cannot specify both flavor_ref and flavor_id") if config[:flavor_id] && config[:flavor_ref]

doctor reports the same conflicts before the run starts, and also catches the inverse — neither set, which is not currently checked anywhere and fails later and less clearly.

The connectivity check is skipped when credentials are missing: "could not authenticate" stacked on "no password is set" is noise, and there is a test pinning that it stays quiet.

Verification

• rake test — 367 examples, 0 failures (360 before; 7 new), rake exit 0
• cookstyle --chefstyle — 11 files, no offenses
• yard stats — 100% documented