Ensure all nested attributes loaded once

- subqueryload_all for each nested attributes ensures that
  attribute is loaded once
- minor refactoring of get admin_interface logic in
  network manager to use sql relations instead of queries each time
- removed yield_per from eagerload methods
  see: http://docs.sqlalchemy.org/en/rel_0_9/orm/query.html#sqlalchemy.orm.query.Query.yield_per
- by deafault collection.eager will use joinedload for each provided field

Change-Id: Ifea46654335bf9ea91835880fbf9d6c96b8b63e7
Closes-Bug: #1328240