CLOUDSTACK-5066: Don't remove the current VPN users when updating

If one VPN user and password is existed in current setup, then don't touch it,
otherwise would result in this user's existing connection be dropped.

systemvm/patches/debian/vpn/opt/cloud/bin/vpn_l2tp.sh

@@ -150,8 +150,12 @@ add_l2tp_ipsec_user() {
    local u=$1
    local passwd=$2
 
-   remove_l2tp_ipsec_user $u
+   uptodate=$(grep "^$u \* $passwd \*$" /etc/ppp/chap-secrets)
-   echo "$u * $passwd *" >> /etc/ppp/chap-secrets
+   if [ "$uptodate" == "" ]
+   then
+       remove_l2tp_ipsec_user $u
+       echo "$u * $passwd *" >> /etc/ppp/chap-secrets
+   fi
 }
 
 rflag=