Skip definitions of built-in loopback models

LoopBack built-in models are special: they follow the loopback-boot
structure and provide `common/models/{name}.json` files, but they are
also automatically loaded (created) by loopback.

This change modifies `executor` to recognize built-in models and do not
redefine them.

This patch is a follow-up for https://github.com/strongloop/loopback/pull/635 and a part of https://github.com/strongloop-internal/scrum-loopback/issues/34

/to @ritch please review