Fix use_single_default_gateway

With FlatDHCP and use_single_default_gateway nova-network creates
an invalid entry in the dnsmasq .opts file for the network that
contains a blank router option.  New instances will not receive
a default gateway.

Change-Id: I4c2da61def0681b7b253ae387bf83c069484e28b
Fixes: bug #1075859
(cherry picked from commit 1b455923446de6fcfee7166059db72b80ff51dfe)
Merge "Fix use_single_default_gateway" into stable/essex
Merge branch 'essexport' of https://github.com/NetApp/nova into essexport
BUGFIX: #1095633 Netapp driver: repeat DFM lun refresh

Signed-off-by: Rushi Agrawal <rushi.agr@gmail.com>