fix(emqx): increase disk to 6GB and add optional MQ disable prompt (#11844)

EMQX 6.1+ preallocates significant disk space for the MQ feature,
causing high CPU/disk usage on small containers (emqx/emqx#16649).

- Increase default disk from 4GB to 6GB
- Add read -rp prompt during install to optionally disable MQ feature
  via mq.enable=false in emqx.conf (reduces disk/CPU overhead)
- Setting is in install script (not CT script) per reviewer feedback

Co-authored-by: sim-san <sim-san@users.noreply.github.com>

ct/emqx.sh

@@ -9,7 +9,7 @@ APP="EMQX"
 var_tags="${var_tags:-mqtt}"
 var_cpu="${var_cpu:-2}"
 var_ram="${var_ram:-1024}"
-var_disk="${var_disk:-4}"
+var_disk="${var_disk:-6}"
 var_os="${var_os:-debian}"
 var_version="${var_version:-13}"
 var_unprivileged="${var_unprivileged:-1}"