Neutron does not follow the RFC 3442 spec for DHCP

When setting a gateway and additional host routes in neutron subnet, the
gateway is only sent to clients via the router dhcp option, dhcp clients
conforming to rfc3442 will ignore router option if
classless-static-routes are available. This patch ensures setting both
the router option and the classless-static-routes including the gateway

Change-Id: Ia00b9385025020f848872309ae42ddac08528f53
Closes-Bug: #1317935
(cherry picked from commit ce59e63249dfcd86e782d056407e38e845cbe19c)
Merge "Neutron does not follow the RFC 3442 spec for DHCP" into stable/icehouse