Steps execution via Connect()

What does this MR do?

Existing Steps execution is quite Docker specific.

• This moves some of the common elements out to the steps package
• Moves the Docker specific part to adhere to the Connect() interface, where we return a io.ReadWriteCloser, representing a connection to a step-runner.

Why was this MR needed?

• Eventually, other executors will implement Connect()
• Paves the way to use Connect() to execute steps in other contexts, other than just our existing run:` scenario.

What's the best way to test this MR?

The existing functionality was rely mostly on integration tests. I think for this MR we should also rely on them, and then increase test coverage as we refactor to accomodate additional scripts-to-steps.

What are the relevant issue numbers?

Closes step-runner#288