Add host_routes property to Neutron subnet resource

Template snippet that was functionally tested:

subnet:
  Type: OS::Neutron::Subnet
  Properties:
    host_routes:
    - {destination: 192.168.1.0/24, nexthop: 192.168.1.2}
    network_id: {Ref: network}
    ip_version: 4
    cidr: 10.0.3.0/24
    allocation_pools:
    - {end: 10.0.3.150, start: 10.0.3.20}

(Next hop probably would make more sense to be in the CIDR)

Change-Id: I36f72f006a1ba48a6ed55a1c361b3df8d1359caa
Closes-Bug: #1250710
Merge "Add host_routes property to Neutron subnet resource"