Add an AvailableNodeManagers model to start making NodeManager API driven

This pull request also includes a perform endpoint in the API that allows direct invocation of exposed actions.
I find this class name confusing and think that we may want to clarify it.  The name implies that it would be a filter of node_managers instead of a pool.  Something like managers would be clearer here.  Then the relationship between a manager and a node_manager would be more intuitive.