Fix bug related to metadata fix for DE513

Due to improper logic in the patch, if the
router wasn't plugged into the floating network
and therefore had no qg interface, interface_name would
come back empty and throw an exception when attempting
to build the iptables rules.

Not-in-upstream: true
Closes-rally-bug: DE860
Closes-rally-bug: DE513
Merge pull request #33 from CiscoSystems/de860

Fix bug related to metadata fix for DE513