Orka SSH executor & integration test

What does this MR do?

This reorganizes some of the code in !124 (closed) & !125 (closed) to be merged before deciding on the authentication story in !123 (merged). This is fine because we don't plan on executing customer builds for a while.

• Implement the SSH Executor.
• Add integration test for the Orka provider & ssh executor. This integration test will run a dummy build in our Orka cluster and ensure it runs as expected.

This necessitates an update of our CI image to install Openconnect since Alpine doesn't have it. I moved it to a base ubuntu image so we can manage our own dependencies and don't depend on the go image updating their own base OS (see gitlab-org/gitlab-runner#27279)

What's the best way to test this MR?

This code is quite difficult to unit test, so I am adding a new integration test that exercises the whole flow.

Execute scripts/orka_integration/run.sh following the instructions in the header

What are the relevant issue numbers?

Issue: #35 (closed)