Cause: The default postfix configuration uses `inet_interfaces = localhost` which
tells postfix to listen on all interfaces resolving to `localhost` including
both IPv4 and IPv6 interfaces.

Consequence: If IPv6 is disabled on the host, postfix and command line tools
such as postconf will issue an error, and the role will fail.

Fix: Using `postconf -h default_database_type`, see if postconf fails with the
error message that indicates IPv6 is disabled. If so, then set `inet_protocols = ipv4`
so that postfix will only use the IPv4 interface.

Result: The postfix role works when IPv6 is disabled.

Adds a new test tests_disable_ipv6.yml to check for this.

Signed-off-by: Rich Megginson <rmeggins@redhat.com>

1. 1. Summary by Sourcery

Detect IPv6-disabled environments and configure postfix to use only IPv4, streamline configuration management, and add an integration test for IPv6-disabled hosts.

New Features:

• Auto-detect when IPv6 is disabled and set postfix to listen only on IPv4
• Add a test playbook to validate the postfix role on an IPv6-disabled host

Enhancements:

• Consolidate configuration application by delegating to manage_config.yml

Tests:

• Introduce tests_disable_ipv6.yml to verify role behavior with IPv6 disabled