Invalid configuration on deploy can make it into the state

[jandeschuttere]

Hi

A couple of days ago I had opened a PR without paying attention to the contributing doc, appologies.

We've found an issue on deploy. If the configuration of the proxy would be invalid - in our case this was a change in how the certificates are being registered - the error returned at the boot of the newly configured proxy would prevent further rollout. Yet this error would not prevent the state to already have been updated with the faulty configuration.

In the event of a restart of the proxy before an updated deploy was rolled out this would prevent the system to be able to get up and running.

Hope you agree with the taken approach, I've kept the impact minimal and added a covering test case.

[kevinmcconnell]

Thanks @jandeschuttere! I think in the case of a legit bug like this it's totally fine to go straight to a PR. The contributing doc is trying to help avoid people from putting time into things that would be better solved through discussion. But bugs need to be fixed, so it's all good :)

I've been heads down on some other projects recently, hence the delay, but I'll take a look at your PR shortly!