`pycloudlib.instance.BaseInstance._wait_for_system` assumes `cloud-init status --wait` will exit zero

`cloud-init status --wait` will exit non-zero if there were errors running cloud-init. As we are now using `pycloudlib` for cloud-init's integration testing (where we will want to see our test _assertions_ fail while developing them, and may want to launch "broken" instances to test recovery), this should no longer be considered a hard failure.

This is the code in question: https://github.com/canonical/pycloudlib/blob/fcf6268df72540b99872c1312b8716a7e88b7564/pycloudlib/instance.py#L363-L367

(On focal, at least, the `runlevel` fallback doesn't trigger because `$(runlevel)` gives `N 5`.)