Fix error messages when installing iptables-persistent

Change-Id: Ib844d9a927f61fc4018d87905701564660f38ad2
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/installers/full_install_osm.sh b/installers/full_install_osm.sh
index e759c56..0f8b177 100755
--- a/installers/full_install_osm.sh
+++ b/installers/full_install_osm.sh
@@ -229,7 +229,7 @@
 function nat(){
     echo -e "\nChecking required packages: iptables-persistent"
     dpkg -l iptables-persistent &>/dev/null || ! echo -e "    Not installed.\nInstalling iptables-persistent requires root privileges" || \
-    sudo DEBIAN_FRONTEND=noninteractive apt-get -yq install iptables-persistent
+    sudo apt-get -yq install iptables-persistent
     echo -e "\nConfiguring NAT rules"
     echo -e "   Required root privileges"
     sudo $OSM_DEVOPS/installers/nat_osm
@@ -721,7 +721,7 @@
 function juju_createproxy() {
     echo -e "\nChecking required packages: iptables-persistent"
     dpkg -l iptables-persistent &>/dev/null || ! echo -e "    Not installed.\nInstalling iptables-persistent requires root privileges" || \
-    sudo DEBIAN_FRONTEND=noninteractive apt-get -yq install iptables-persistent
+    sudo apt-get -yq install iptables-persistent
 
     if ! sudo iptables -t nat -C PREROUTING -p tcp -m tcp --dport 17070 -j DNAT --to-destination $OSM_VCA_HOST; then
         sudo iptables -t nat -A PREROUTING -p tcp -m tcp --dport 17070 -j DNAT --to-destination $OSM_VCA_HOST