Handle context cancelations for SSH executor

⚠ depends on !148 (merged)

What does this MR do?

Allows a context to be passed through dial, key exchange and script run functions allowing cancelation at any time.

Why was this MR needed?

The SSH executor was passed a context object that it was not using.

What's the best way to test this MR?

There's unit and integration tests added.

Manual QA:

1. Create a job with the SSH executor
2. Cancel the job.
3. Check cancelation happened immediately? WIP: How do we want to handle the script cancelation here?

What are the relevant issue numbers?

Closes #70 (closed)