Add a route to reach the MD server when a subnet is created

Add a route to reach the MD server when a subnet is created

When the first subnet is created, the dhcp port is created and
midonet plugin correctly adds the static route to reach the MD
server in create_port. When a second or following subnets are
created, a new ip is added to the dhcp port. This patch takes
care of adding the static route to correcly reach the MD server
in update_port. This fixes the problem of VMs not being able to
reach the MD if assigned to the second subnet

Closes-bug: #1231914
Change-Id: Ifc95f901d4222b76a4254e21295829ac8d82493b