Add network connectivity check for cluster install/join

[vicaya]

In many environments, including cloud (esp. oracle cloud), instances are isolated by os and platform firewall/network security policies/rules that only allow port 22 (ssh), which means cluster install/join would fail with cryptic errors e.g. #429.

Suggest adding a connectivity check to master port 6443 in k3sup join. Upon timeout, warn users about firewall issues that need to be addressed.