Use eventlet executor in rpc_service
The default executor of o.m is 'blocking', but after change ,
o.m with blocking is broken. 'eventlet' and 'non-blocking' work
fine and most services use 'eventlet', so this is why it was
noticed by magnum which used the default one.
Use eventlet to not be affected by bug #1694728 , but more
importantly, the oslo team suggests to not use blocking which
has 0% test coverage and it is going to be deprecated and unset
from the default configuration.
(cherry picked from commit