os-net-config fails to validate dns_servers value when it contains single IP address

Issue

• Undercloud deployment failed after user defined net_config_override in undercloud.conf and set network_config with single IP address definition for dns_servers.

• Undercloud deployment failed because of the following error after user defined net_config_override in undercloud.conf and set network_config with single IP address definition for dns_servers:

  2023-06-27 11:27:00.142365 | 00505684-f5f6-0ad3-efe2-0000000008d8 |       TASK | Ensure firewall is installed
  2023-06-27 11:28:43.733569 | 00505684-f5f6-0ad3-efe2-0000000008d8 |      FATAL | Ensure firewall is installed | director | error={"changed": false, "msg": "Failed to download metadata for repo 'rhel-9-for-x86_64-highavailability-eus-rpms': Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried", "rc": 1, "results": []}