Neutron agent-list shows agents status running on compute and network nodes.  If you run this command to check agent list after your OpenStack and it returns empty then there is some thing wrong with configuration at compute or network nodes.

you better check logs first, i hit this error on network node so i'll concentrate on network node only, so as we are discussing, logged into my network node and started monitoring my neutron and tailed all agent logs.
root@network:~# cd /var/log/neutron/
root@network:/var/log/neutron# tail -f *agent*
the moment i hit enter i was flooded with logs from l3-agent.log. 
==> l3-agent.log <==
2015-08-20 23:47:53.338 18029 TRACE neutron.openstack.common.service   File "/usr/lib/python2.7/dist-packages/neutron/openstack/common/service.py", line 184, in _wait_for_exit_or_signal
2015-08-20 23:47:53.338 18029 TRACE neutron.openstack.common.service     rpc.cleanup()
2015-08-20 23:47:53.338 18029 TRACE neutron.openstack.common.service   File "/usr/lib/python2.7/dist-packages/neutron/openstack/common/rpc/__
init__.py", line 208, in cleanup
2015-08-20 23:47:53.338 18029 TRACE neutron.openstack.common.service     return _get_impl().cleanup()
2015-08-20 23:47:53.338 18029 TRACE neutron.openstack.common.service   File "/usr/lib/python2.7/dist-packages/neutron/openstack/common/rpc/__
init__.py", line 274, in _get_impl
2015-08-20 23:47:53.338 18029 TRACE neutron.openstack.common.service     _RPCIMPL = importutils.import_module(impl)
2015-08-20 23:47:53.338 18029 TRACE neutron.openstack.common.service   File "/usr/lib/python2.7/dist-packages/neutron/openstack/common/import
utils.py", line 57, in import_module
2015-08-20 23:47:53.338 18029 TRACE neutron.openstack.common.service     __import__(import_str)
2015-08-20 23:47:53.338 18029 TRACE neutron.openstack.common.service ImportError: No module named rabbit
2015-08-20 23:47:53.338 18029 TRACE neutron.openstack.common.service
all it shouting at me was that there was no module called "rabbit" so i started reading a little about rabbitMQ then i found a huge reference guide from openstack. It explains configuring RPC messaging system using different message brokers like RabbitMQ, Qpid, and ZeroMQ.

It shows how to configure “rpc_backend” in neutron.conf file. Openstack installation guide says "rpc_backend = rabbit” but this guide says “rpc_backend=neutron.openstack.common.rpc.impl_kombu” which is correct and works fine.

so i modified /etc/neutron/neutron.conf as mention in this doc and the restart neutron-l3-agent since it was the only one failing. take a look at the l3-agent.log after restart 
root@network:/var/log/neutron# tail -f l3-agent.log
==> l3-agent.log <==
2015-08-21 01:30:50.236 10786 INFO neutron.openstack.common.rpc.common [req-9b23691a-cbce-41ab-a98e-686fb0473475 None] Connected to AMQP server on controller:5672
2015-08-21 01:30:50.238 10786 INFO neutron.openstack.common.rpc.common [req-9b23691a-cbce-41ab-a98e-686fb0473475 None] Connected to AMQP server on controller:5672
2015-08-21 01:30:50.265 10786 INFO neutron.openstack.common.rpc.common [req-9b23691a-cbce-41ab-a98e-686fb0473475 None] Connected to AMQP server on controller:5672
2015-08-21 01:30:50.282 10786 INFO neutron.agent.l3_agent [req-9b23691a-cbce-41ab-a98e-686fb0473475 None] L3 agent started
2015-08-21 01:31:01.903 10786 INFO neutron.openstack.common.service [req-9b23691a-cbce-41ab-a98e-686fb0473475 None] Caught SIGTERM, exiting
2015-08-21 01:31:02.721 10835 INFO neutron.common.config [-] Logging enabled!
2015-08-21 01:31:02.843 10835 INFO neutron.openstack.common.rpc.common [req-7c918e45-aad3-4a20-b303-0d3a8964be97 None] Connected to AMQP server on controller:5672
2015-08-21 01:31:02.846 10835 INFO neutron.openstack.common.rpc.common [req-7c918e45-aad3-4a20-b303-0d3a8964be97 None] Connected to AMQP server on controller:5672
2015-08-21 01:31:02.868 10835 INFO neutron.openstack.common.rpc.common [req-7c918e45-aad3-4a20-b303-0d3a8964be97 None] Connected to AMQP server on controller:5672
2015-08-21 01:31:02.872 10835 INFO neutron.agent.l3_agent [req-7c918e45-aad3-4a20-b303-0d3a8964be97 None] L3 agent started
wonderful it is connected to rabbitMQ server successfully.

gil ...