eventlet: stop monkey patching

Let's stop monkey patching Python internals. It creates a lot of problem
(cf SQLAlchemy pool issues fixed by
64bc862f6fa936dbe5dd2d402240a81da40bc5a5) as nothing has ever been
designed with multi-threading in mind. It is incompatible with a lot of
library using threads internally, causing random issues like dead-locks.

Change-Id: Iad37c418be4780a7e78bf48fa080dda4ae1035b8