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This has support for generating arguments to AddNodes() and
DeleteNodes(), which means re-reading the current manifest to know what
the original parameters were, so that the appropriate args (like which
image, which lans to join, etc) go to AddNodes().

Also, adds a geni-lib script that generates an rspec instead of printing
it (although print still works at portal) and generates input for
CM::AddNodes() when requested.  This generator is stateful; it tries
to avoid generating new nodes with previously-used IPs or client_ids;
thus it is a separate object.  It is designed so that it can be imported
into a script, and the importing script can look for special
DYNSLICE_GENERATOR variables to use its rspec foo to create a slice and
add nodes in some semantic way.