Use hostname for report-host instead of ip in ipv6 environments

[Federico Bosi]

Deploying the charm in an ipv6 only environment fails during bootstrap, saying that the report host option is invalid.

Juju logs:
unit-vault-mysql-router-1: 09:35:39 INFO unit.vault-mysql-router/1.juju-log shared-db:103: Invoking reactive handler: reactive/mysql_router_handlers.py:39:bootstrap_mysqlrouter
unit-vault-mysql-router-1: 09:35:39 ERROR unit.vault-mysql-router/1.juju-log shared-db:103: Failed to bootstrap mysqlrouter: Error: Option --report-host has an invalid value.

The official docs state "Before 8.0.23, validation checked the hostname string for illegal characters where only alphanumeric, '-', '.', and '_' characters were allowed. For example, this meant that IPv6 addresses were not allowed."

Though I don't see this behavior:

ubuntu@giving-seal:~$ mysqlrouter --version
MySQL Router Ver 8.0.36-0ubuntu0.22.04.1 for Linux on x86_64 ((Ubuntu))
ubuntu@giving-seal:~$ mysqlrouter --report-host my-prefix:142:0:1:20
Error: Option --report-host has an invalid value.
ubuntu@giving-seal:~$ mysqlrouter --report-host "my-prefix:142:0:1:20"
Error: Option --report-host has an invalid value.
ubuntu@giving-seal:~$ mysqlrouter --report-host "[my-prefix:142:0:1:20]"
Error: Option --report-host has an invalid value.
ubuntu@giving-seal:~$ mysqlrouter --report-host [my-prefix:142:0:1:20]
Error: Option --report-host has an invalid value.

I'll eventually submit a bug to mysql devs, meanwhile I fixed this by using charmhelpers get_hostname which in my MAAS environment successfully returns a valid hostname and the router is able to bootstrap.
I thought it could be a good idea to check the version and use the hostname if it's <8.0.23, but since it seems broken also on >8.0.23 maybe we can put a warning that deploying "will" fail in case we can't find a valid hostname?

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/charm-mysql-router/+/916114