[r=blake-rouse][bug=][author=mpontillo] Network interfaces constraint refactoring, part 2.
* Enable basic "interfaces" constraints for Node.acquire() API.
* Does not yet support matching each constraint back to its interface.
* Extend framework for specifier-based queries into the Interface manager.
* Extend the generic query-by-specifiers functionality to support specifiers other than functions. (specifier aliases, Django query strings, and references to a different model object's query-by-specifiers are now supported.)
* Make the superclass more robust to handle collaboration between filtering entities.
* Support for (the positive form of) the following specifiers for interfaces: subnet, space, subnet_cidr, fabric, fabric_class, id, vid, and name.