Fix config-file overrides for nova-dhcpbridge
Fix config-file overrides for nova-dhcpbridge

nova-dhcpbridge can read in an alternate config file location but wasn't
using it to parse config options.  This fixes a regression introduced in
00786bc554a2dfacb3c6f02fbb7e9c98f35d4262.

This change is motivated by the fact that nova-dhcpbridge is passed to
dnsmasq in restart_dhcp() in nova/network/linux_net.py, and there is
currently no support for overriding config options for nova-dhcpbridge
in that call.  There is support for overriding the FLAGFILE env variable
in that call, but nova-dhcpbridge was ignoring that.  If I start
nova-network with an alternate config this provides a way for me to pass
that config to nova-dhcpbridge as well.

Change-Id: I002ea768500555a089bd1d5b0fea2354b27fba14

n0ano/ganttclient