[PR] Use HTTP for network detection rather than ping

Relying on ping to detect network connectivity was troublesome assome networks block icmp and would generate a false error. As we need HTTP access throughout provisioning anyway, we can rely on it as an indicator of network health.

This ends up being quite a bit faster too. We could probably get even better with just a DNS request, but that may hide another issue depending on the network.

Fixes #428 - thanks to @kazurayam for raising the issue originally.