Fix OS-123, support for gc-install-agent on instances with multiple IPs.

Tested on an ATT SilverLining instance:
f626b14c-fc81-4e94-a1cb-c9bda643e8ef | first test | ACTIVE | vlan2019=10.0.19.3, 64.143.228.34

The 10.* address is unreachable from us, the 64.* address is a floating IP.

Change-Id: I3eac69858f2111d467faa6b3deb126d5c82d7bb5