Fix iscsi_ip_address to work automatically for AIO

In https://review.openstack.org/#/c/101589/ a change was made to allow compute nodes to connect to iscsi volumes that don't reside on the same physical host. However, the default value used is the IP address on eth1, which causes a minor headache for many people deploying AIO with a single NIC (usually eth0). We should add a bit to the install.sh script to set the value properly for the AIO case.