executor: run experiment on an executor

[dberenbaum]

Meta-issue. Please link related tickets back to this one.

User story

I want to run an experiment on my executor from my local machine.

Requirements

• dvc exp run --executor successfully runs an experiment on a running executor instance with no other steps.

Out of scope

• It's not yet necessary to monitor the experiment or its output while it runs or to be notified when the experiment completes.
• It's not yet necessary to queue or run multiple experiments on an executor.
• It's not yet necessary to create or destroy the executor instance as part of the experiment run.

[dberenbaum]

It's important to keep the remote executor's experiment running even if the connection from the local machine breaks.

May need a dvc exp collect or similar command to check status of running experiments.

[dberenbaum]

Need to rescope