NTP charm is not subnet-aware & picks non-accessible peer addresses

[Paul Gear]

If the units on which the NTP subordinate charm is installed are multi-homed, the charm does not always pick the right interface for each peer, and sometimes ends up picking unreachable unit addresses: https://pastebin.ubuntu.com/p/zYQ3rmN8DR/

The charm should either test and use only accessible addresses, or use all unit addresses.

[Stuart Bishop (stub)]

Juju 2.3 provides specific bind and ingress addresses to solve this sort of issue.

Some sort of fallback will of course be needed if we need to support older deployments.

Machines added with the manual provider may be a wildcard here, since their networking is not under Juju control. 'juju relate ... via ...' might help here (https://docs.jujucharms.com/2.3/en/developer-network-primitives), or an extra-binding (https://docs.jujucharms.com/2.3/en/authors-charm-metadata) to allow the rage to be specified at deploy time.