Provide routes for neighbor IPv4 subnets

Provide routes for neighbor IPv4 subnets

Network may contain several Subnets.  In this case all these subnets are
accessible via same link and two VMs with addresses from different
Subnets may talk to each other directly, bypassing default router.  For
this to work, "neighbour Subnets" should have entry in VM's routing
tables.  RFC3442 describes this situation:

   In some cases more than one IP subnet may be configured on a link.
   In such cases, a host whose IP address is in one IP subnet in the
   link could communicate directly with a host whose IP address is in a
   different IP subnet on the same link. In cases where a client is
   being assigned an IP address on an IP subnet on such a link, for each
   IP subnet in the link other than the IP subnet on which the client
   has been assigned the DHCP server MAY be configured to specify a
   router IP address of 0.0.0.0.

When network contains more that one IPv4 subnets, report these subnets
in 'classless-static-routes' DHCP option.

DocImpact

Change-Id: Ifcf1d99e0f0136bf52b8d13675b7ccfd48005fab
Closes-Bug: #1372885