Only generate dhparams if needed

src/freedombone-app-vpn

     sed -i "s|export KEY_EMAIL.*|export KEY_EMAIL=\"$MY_EMAIL_ADDRESS\"|g" /etc/openvpn/easy-rsa/vars
     sed -i "s|export KEY_OU=.*|export KEY_OU=\"MoonUnit\"|g" /etc/openvpn/easy-rsa/vars
     sed -i "s|export KEY_NAME.*|export KEY_NAME=\"$OPENVPN_SERVER_NAME\"|g" /etc/openvpn/easy-rsa/vars
-    openssl dhparam -out /etc/openvpn/dh2048.pem 2048
+    if [ ! -f /etc/openvpn/dh2048.pem ]; then
+        openssl dhparam -out /etc/openvpn/dh2048.pem 2048
+    fi
     cd /etc/openvpn/easy-rsa
     . ./vars
     ./clean-all