Fix occasional deletion failure

Stack-delete was failing periodically because of heat bug
https://bugs.launchpad.net/heat/+bug/1299259, whereby there's
an implicit dependency from floating IPs to the router interface.
There is a fix in progress for heat, but until then, this patch
adds an explicit depends_on to the router interface.

Change-Id: I846abf1fbd90d5f5c71b1c13941463743ace48ad
Closes-Bug: 1356721
Merge "Fix occasional deletion failure"