Use reasonable timeout for rpc service_update()

nova.servicegroup.drivers.db.DbDriver._report_state() is called every
service.report_interval seconds from a timer in order to periodically
report the service state. It calls self.conductor_api.service_update().

If this ends up calling nova.conductor.rpcapi.ConductorAPI.service_update(),
it will do an RPC call() to nova-conductor.

If anything happens which causes the RPC reply to be lost or
never sent in the first place, by default the RPC code will
wait 60 seconds for a response (blocking the timer-based
calling of _report_state() in the meantime).
This is long enough to cause the status in the database to get old
enough that other services consider this service to be "down".

if rpc_reponse_timeout is smaller than report_interval then we could
use the existing RPC timeout, but wait longer won't hurt. So the patch
didn't check it and only use report_interval.

Change-Id: I88743183bce1a534812cfe6110c3fc2892058c53
Closes-Bug: #1368989
Merge "Use reasonable timeout for rpc service_update()"