The loopback driver implements cloud.Driver by presenting a fake cloud in which

• 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

Using the loopback driver will involve some special configuration.

• 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.