[arvados-dispatch-cloud] loopback driver
- Instances() returns a single instance named "localhost"
- Create() fails
- Destroy() fails
- Instance address points to an SSH server (brought up by the driver) that accepts the dispatcher's key and executes shell commands
- InstanceTypes should be configured with a single instance type, with the host's RAM/CPU specs
When combined with #14922 this should make crunch-dispatch-local redundant.
This will also facilitate an arvados-dispatch-cloud integration test that uses the real crunch-run program instead of a stub. This might involve a few other changes, like a configurable location for lockfiles.