⭕ Retry if Servers, SSH Keys, or Load Balancer creation returns 500 response

[oscarhermoso]

This is a workaround for an issue with the create endpoints - sometimes they return a 500 response. See #13

Explanation from BL:

I've reviewed the logs and it's a problem on our side when a user hits the API quickly after a period of inactivity. Actual fix is simple, but deploying it won't be trivial, so can't be sure where it will fit into the pipeline of new stuff.

This change also adds test sweepers, to clean up any lingering resources that are left around when the test suite breaks.