distro/rhel86: disable pw auth for EC2 images

Disable loging in via password authentication since this is an
official Amazon marketplace requirement

  Linux-based AMIs must not allow SSH password authentication.
  Disable password authentication via your sshd_config file by
  setting PasswordAuthentication to NO.

  Section "Security policies" from
  https://docs.aws.amazon.com/marketplace/latest/userguide/product-and-ami-policies.html

test/data/manifests/centos_8-x86_64-ami-boot.json

@@ -1015,6 +1015,14 @@
               "profile": "sssd"
             }
           },
+          {
+            "type": "org.osbuild.sshd.config",
+            "options": {
+              "config": {
+                "PasswordAuthentication": false
+              }
+            }
+          },
           {
             "type": "org.osbuild.fstab",
             "options": {