RSpecs: ExoSM Ad includes external references

[ahelsing@bbn.com]

ExoSM (not individual racks) advertisement RSpec should include external references - show links to non-ExoGENI resources. EG to the mesoscale, NLR, Internet2, etc.

These connections should follow the stitching guidelines:

-there must be a link element for each connection to an External Aggregate.
-these link elements will each have two interface_ref component_id
-one of the interface_ref component_id references a local node->interface component_id
-the other interface_ref component_id is equal to a  link--->interface_ref component_id  in a link element of the External Aggregate 
-both of the peering Aggregates need to point to each other in consistent manner within their mainBody:link elements

Note the relation to ticket #37