imporve exception handles inside spawn

[Zang MingJie]

eventlet pool will discards all exception raised inside spawn(_n), it is hard to discover problems inside spawned green-threads, it is better to add a function wrapper to log the exceptions raised inside spawned function.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/106362

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/106362
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=7d869a02b8245dbf9d22ddf73ff810d055581730
Submitter: Jenkins
Branch: master

commit 7d869a02b8245dbf9d22ddf73ff810d055581730
Author: Zang MingJie <email address hidden>
Date: Wed Jul 16 15:41:45 2014 +0800

    Log exceptions inside spawned functions

    eventlet pool will discards all exception raised inside spawn(_n), it is
    hard to discover problems inside spawned green-threads, it is better to
    add a function wrapper to log the exceptions raised inside spawned
    function.

    Change-Id: I753df36c4614759f49134667a55f2a91f0c08317
    Closes-bug: #1340708