add '--nat:0.0.0.0' flag to hive_integration/nimbus/nimbus.sh

trying Jacek suggestion in #591, I added nat setting to
nimbus-eth1 hive shell script. visible difference after adding
this flag is the nat library not complaining about
"the gateway does not support nat-pmp" anymore.

but the slow startup time described in #591 is yet to be measured
again, although I already see improvement when executing
ethereum/consensus category in hive.

hive_integration/nimbus/nimbus.sh

@@ -47,7 +47,7 @@
 set -e
 
 nimbus=/usr/bin/nimbus
-FLAGS="--prune:archive"
+FLAGS="--prune:archive --nat:0.0.0.0"
 
 if [ "$HIVE_LOGLEVEL" != "" ]; then
   FLAGS="$FLAGS --log-level:DEBUG"