RSpecs: Rack Ads including proper stitching extension

[ahelsing@bbn.com]

Individual racks (not ExoSM) should include a proper stitching extension ad: include the extension once, with a single aggregate.

Follow Tom Lehman's rules for populating that extension:

-Identify all mainBody:link elements which have a link->interface_ref component_id which is for an External Aggregate.'
-A stitch:node element must be created for each of the nodes associated with the above identified mainBody:link elements
    Note:  only the mainBody:node elements which are part of a connection to an External Aggregate must be included
                in the Stitching Extension.  However, Aggregates may include other mainBody:node elements if they desire.
-The stitch:node elements are populated as follows described below:
   --stitch:node id==mainBody:node component_id
   --stitch:port id==mainBody:link->interface_ref component_id (which equals a node->interface component_id)
   --stitch:link id==stitch:portid with ":some-descriptive-term" appended
   --stitch:remoteLinkId==the stitch:link id from the External Aggregate stitching extension
-both of the peering Aggregates need to point to each other in consistent manner with  their stitch:link and stitch:remoteLinkId elements

[ibaldin@renci.org]

Same as #37. Nothing different in behavior of rack SM vs. ExoSM with respect to these comments.