box: skip 'entering the event loop' for failover

This message doesn't give any useful information for `tarantool
--failover` run, because it always starts background fibers for
monitoring purposes. Let's suppress the message in the case.

Part of https://github.com/tarantool/tarantool-ee/issues/564

NO_DOC=It is a small visual improvement, there is nothing to document
       here.
NO_CHANGELOG=It is part of Tarantool Enterprise Edition feature.
NO_TEST=Doesn't worth it.