In the production sometimes the provision and other networks should be
separated and all have each own different gateways, so this change adds
the correct way to configure bifrost with gateway used only to configure
dnsmasq and solves the issue with several default routes on the bifrost
host when kolla_bifrost_dnsmasq_router needs to configure.

Change-Id: I2078da3ab3898c847b3c17054f429d74eb79aa37
Signed-off-by: Maksim Malchuk <maksim.malchuk@gmail.com>