Work around the "Could not determine host ip address" error when you run stack.sh

For some reasons, I got this error when running the stack.sh script even though I have my local.conf file setup properly (I used the same local.conf for many times)

devstack$ ./unstack.sh
[Call Trace]
./unstack.sh:33:source
/opt/stack/devstack/openrc:27:source
/opt/stack/devstack/stackrc:759:die
[ERROR] /opt/stack/devstack/stackrc:759 Could not determine host ip address. See local.conf for suggestions on setting HOST_IP.

One quick workaround is to set the HOST_IP default value in stackrc:

devtack $ nano stackrc

...
HOST_IP=${HOST_IP:-<my host ip address>}
...

Then stack.sh will run smoothly :)