Add subnet and router details to neutron plugin

Summary
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If neutron provides enough through notifications that we're able to index it in a meaningful way (and it does, see the whiteboard), we should try to.

Requested data at the Mitaka summit is:

  * routers, networks, subnets, ports. Question is how to map it. Separate (as neutron does)? Nested objects? - leaning towards parent/child, since nested will require e.g. on a network change to reindex all attached ports or resort to script-based updating, and make it harder to treat ports as first-class query objects

Motivation
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Ultimately it would be good to be able to search on an IP and get back instance info (and dns, networks etc)

Description
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Testing
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Brief instruction for reviewers to exercise the changes, including expected results where non-obvious.

Outside Dependencies
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Is this functionality already supported in other services? List the appropriate API calls and if they are extensions or base API functionality.

This should describe any cross project dependencies. This should include:
  * changes to OpenStack services
  * changes to OpenStack services clients (python-*client)
  * changes in external projects
Links to particular patches should be included in the whiteboard area.

Requirements Update Required
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