[Nsi-wg] Updated NSI NSA Discovery document

Peoples, Here is the updated document describing the NSA Discovery schema for using with the NSI Discovery protocol. I made one small change to the schema from the version sent two weeks ago. I add the expires attribute in the NsaType definition for compatibility with the new NSI Discovery protocol. Please have a read and provide comments. I will try to get a draft of the NSI Discovery protocol out tomorrow to provide additional context. John