ansible: Smaller changes in common_deb

- use postfix by default
- nftables validation
- socat

ansible/roles/common_deb/defaults/main.yml

 tz: Europe/Prague
 mail_relay: relay.fi.muni.cz
-postfix: false
+postfix: true
 pam_use_kerberos: True
 backports_git: False
 backports: False

ansible/roles/common_deb/tasks/main.yml

@@ -118,6 +118,7 @@
   template:
     src: nftables.conf.j2
     dest: /etc/nftables.conf
+    validate: nft --check -f %s
   notify:
     - restart nftables
   when: nftables_default_config
@@ -259,6 +260,7 @@
       - moreutils
       - bat
       - iotop
+      - socat
 
 - name: monitoring
   apt: