* Auto-join support for IPv6 discovery
The go-discover library returns IP addresses and not URLs. It just so
happens net.URL parses "127.0.0.1", which isn't a valid URL.
Instead, we construct the URL ourselves. Being careful to check if it's
an ipv6 address and making sure it's in explicit form if so.
Fixes#12323
* feedback: addrs & ipv6 test
Rename addrs to clusterIPs to improve clarity and intent
Tighten up our IPv6 address detection to be more correct and to ensure
it's actually in implicit form
Daniel Kimsey