Faster dhparams for vpn

src/freedombone-app-vpn

 function vpn_generate_keys {
     # generate host keys
     if [ ! -f /etc/openvpn/dh2048.pem ]; then
-        openssl dhparam -out /etc/openvpn/dh2048.pem 2048
+        ${PROJECT_NAME}-dhparam -o /etc/openvpn/dh2048.pem
     fi
     if [ ! -f /etc/openvpn/dh2048.pem ]; then
         echo $'vpn dhparams were not generated'