Fix floating-ips in error state in dvr mode

Before this fix an exception was raised when
creating the veth between the fip namespace and
the qrouter namespace when the veth was already
present.
This fix add a check to only create the veth if
not present.

Change-Id: Iefea9778223aac885bedfdacfdeacfce74776333
Closes-Bug: #1376013
Merge "Fix floating-ips in error state in dvr mode"