Globally redirect stdout to stderr, except when echo'ing "export HOSTS=...".
Globally redirect stdout to stderr, except when echo'ing "export HOSTS=...".

This should make development easier by allowing developers to set HOSTS in their calling shell, and re-use the value for this env. var. when calling individual test scripts, e.g.:
```
cd test
eval $(./run-integration-tests.sh setup)
./200_dns_test.sh
```

weaveworks/weave,weaveworks/weave,weaveworks/weave,weaveworks/weave