User should not specify more than 5 DNS nameservers per subnet

When deploying sunbeam on some platforms, you may see the following DNSNameServersExhausted exception [0] when using 'sunbeam configure --accept-defaults --openrc demo-openrc' in a fresh deployment.

Error: Error creating openstack_networking_subnet_v2: Bad request with: [POST http://10.20.21.10:80/openstack-neutron/v2.0/subnets], error message: {"NeutronError": {"type": "DNSNameServersExhausted", "message": "Unable to complete operation for new subnet. The number of DNS nameservers exceeds the limit 5.", "detail": ""}}

For example, this issue can be encountered on the stsstack test bed, because it has more than 5 DNS nameservers.

ubuntu@juju-5d90c3-sunbeam-0:~$ sudo resolvectl status ens3 |grep 'DNS Server'
Current DNS Server: 10.5.0.15
       DNS Servers: 10.5.0.15 10.5.0.4 10.5.0.3 10.5.0.12 10.5.0.11 10.5.0.17

while the default max_dns_nameservers for neutron [1] is 5, so the exception DNSNameServersExhausted was thrown.

sunbeam is reading nameserver from the file /run/systemd/resolve/resolv.conf according to this commit [2], I think the function get_nameservers inside it should not fectch more than 5 DNS nameservers per subnet to avoid this exception.

[0] https://paste.ubuntu.com/p/8Q4HvJDSV4/
[1] https://docs.openstack.org/neutron/latest/configuration/neutron.html
[2] https://github.com/openstack-snaps/snap-openstack/commit/7b7ca702efb490f13624002093e1b0b4cefe3aab