add network info to topology

add network info to topology

we need to know which network each port is connected to
so we can find VLAN or VXLAN ID.

To do this we implement a new method for Contexts,
Context.get_network().  This method is similar to
Context.get_server(), it searches for a given
network name in all the contexts.

From this we generate a context_cfg["networks"]
dict that stores all the network info for the nodes
in the scenario.

Then when we generate the topology for VNFD, we can
lookup a given network by the vld_id and get the
network_type, segmentation_id, etc.

Then if we need to for example generated
traffic on a given VLAN or VXLAN, we have this
info available.

Define default nd_route_tbl for ACL VNF
we need default empty nd_route_tbl for IPv6 route.

Change-Id: I9f9cfbd6acabeb4ae4675ca7354390efa57b29e7
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Edward MacGillivray <edward.s.macgillivray@intel.com>