From the logs:
2019-06-04T12:58:26.200Z [ERR] Handling panic: invalid argument to Intn – rest.PanicHandler.func1() at config.go:1191
There are two issues worth looking into here. There's the bug itself, which seems like its most likely to be related to either gocb or SG code during random node selection for design doc creation (see capiEps and mgmtEps). It's possible this is just a race, where the value gets updated in between the time we check for it to be zero and the time we call rand.Intn, or it could be related to us picking up changes in gocb to return a value like -1.
If this turns out to be in SG code, it's worth taking another look at the gocb code to see if we can use the SDK to issue this call, instead of building/executing it ourselves. I believe the original motivation was down to the SDK not supporting IndexXattrOnTombstones at the time.
Another concern, though, is why the full stack trace for the panic isn't making it into the logs. This is being caught, logged, then a new panic thrown, but the new panic doesn't get logged anywhere we can use it. That's going to hamper troubleshooting more generally, and should be addressed.