Add support for nexus sub plugin

Support the nexus sub plugin by:

adding an option to site.pp to allow
the nexus_plugin config option to be
set in cisco_plugins.ini

Creating a nexus.ini config file based
on a hash created by the user in site.pp
describing the set of switches, which
nodes are connected to those switches,
and which ports the nodes are using.

Changing the plugin variables to user
readable strings and then converting
as needed in core.pp
Merge pull request #112 from michaeltchapman/nexus_plugin

Nexus plugin