made service reconfiguration better, updated unit tests
slight fix to update_node_pool, unit tests for update_node_pool
added clear_extra for removing instances when config.count decreases
removed ServiceCollection.add, moved .get_instance call to Service instead of ServiceInstanceCollection
factored _build into an update_func for collections.add
improved _build, update_node_pool to be more pythonic
Merge pull request #263 from Codeacious/jrm_node_reconfigure_fix

Made service reconfiguration slightly better