Change nexus_dict to accept port lists

Change nexus_dict to accept port lists

When users configured a server to have two logical connections to a
single switch, the nexus driver would never know about the second
connection because the nexus_dict stored the interface as a single
value, which would just get overwritten.  This has been fixed by
allowing the port value to be a comma seperated list.  All
operations on ports have been update to loop over multiple ports
per switch. New test added for host with dual connections to one
switch.

Change-Id: Iefb30452083747b45496600c81f8d0a6f378bd08
Closes-Bug: 1288393