ovs_dead, ovs-agent booting up code should exit graceful

Currently, ovs-agent doesn't check ovs_dead during its boot up process. That leads to an ungraceful exit. We should avoid the crash and handle that situation graceful.

Stderr: '2014-08-01T03:50:10Z|00002|reconnect|WARN|unix:/var/run/openvswitch/db.sock: connection attempt failed (No such file or directory)\novs-vsctl: unix:/var/run/openvswitch/db. sock: database connection failed (No such file or directory)\n'
2014-08-01 11:50:10.425 17754 CRITICAL neutron [-] RuntimeError:
Command: ['sudo', '/usr/local/bin/neutron-rootwrap', '/etc/neutron/rootwrap.conf', 'ovs-vsctl', '--timeout=10', '--', 'set-fail-mode', 'br-int', 'secure']
Exit code: 1
Stdout: ''
Stderr: '2014-08-01T03:50:10Z|00002|reconnect|WARN|unix:/var/run/openvswitch/db.sock: connection attempt failed (No such file or directory)\novs-vsctl: unix:/var/run/openvswitch/db. sock: database connection failed (No such file or directory)\n'
2014-08-01 11:50:10.425 17754 TRACE neutron Traceback (most recent call last):
2014-08-01 11:50:10.425 17754 TRACE neutron File "/home/bengo/Downloads/pycharm-3.4.1/helpers/pydev/pydevd.py", line 1733, in <module>
2014-08-01 11:50:10.425 17754 TRACE neutron debugger.run(setup['file'], None, None)
2014-08-01 11:50:10.425 17754 TRACE neutron File "/home/bengo/Downloads/pycharm-3.4.1/helpers/pydev/pydevd.py", line 1226, in run
2014-08-01 11:50:10.425 17754 TRACE neutron pydev_imports.execfile(file, globals, locals) # execute the script
2014-08-01 11:50:10.425 17754 TRACE neutron File "/usr/local/bin/neutron-openvswitch-agent", line 10, in <module>
2014-08-01 11:50:10.425 17754 TRACE neutron sys.exit(main())
2014-08-01 11:50:10.425 17754 TRACE neutron File "/opt/stack/neutron/neutron/plugins/openvswitch/agent/ovs_neutron_agent.py", line 1644, in main
2014-08-01 11:50:10.425 17754 TRACE neutron agent = OVSNeutronAgent(**agent_config)
2014-08-01 11:50:10.425 17754 TRACE neutron File "/opt/stack/neutron/neutron/plugins/openvswitch/agent/ovs_neutron_agent.py", line 201, in __init__
2014-08-01 11:50:10.425 17754 TRACE neutron self.setup_integration_br()
2014-08-01 11:50:10.425 17754 TRACE neutron File "/opt/stack/neutron/neutron/plugins/openvswitch/agent/ovs_neutron_agent.py", line 796, in setup_integration_br
2014-08-01 11:50:10.425 17754 TRACE neutron self.int_br.set_secure_mode()
2014-08-01 11:50:10.425 17754 TRACE neutron File "/opt/stack/neutron/neutron/agent/linux/ovs_lib.py", line 128, in set_secure_mode
2014-08-01 11:50:10.425 17754 TRACE neutron check_error=True)
2014-08-01 11:50:10.425 17754 TRACE neutron File "/opt/stack/neutron/neutron/agent/linux/ovs_lib.py", line 71, in run_vsctl
2014-08-01 11:50:10.425 17754 TRACE neutron if not check_error:
2014-08-01 11:50:10.425 17754 TRACE neutron File "/opt/stack/neutron/neutron/openstack/common/excutils.py", line 82, in __exit__
2014-08-01 11:50:10.425 17754 TRACE neutron six.reraise(self.type_, self.value, self.tb)
2014-08-01 11:50:10.425 17754 TRACE neutron File "/opt/stack/neutron/neutron/agent/linux/ovs_lib.py", line 65, in run_vsctl
2014-08-01 11:50:10.425 17754 TRACE neutron return utils.execute(full_args, root_helper=self.root_helper)
2014-08-01 11:50:10.425 17754 TRACE neutron File "/opt/stack/neutron/neutron/agent/linux/utils.py", line 77, in execute
2014-08-01 11:50:10.425 17754 TRACE neutron raise RuntimeError(m)
2014-08-01 11:50:10.425 17754 TRACE neutron RuntimeError:
2014-08-01 11:50:10.425 17754 TRACE neutron Command: ['sudo', '/usr/local/bin/neutron-rootwrap', '/etc/neutron/rootwrap.conf', 'ovs-vsctl', '--timeout=10', '--', 'set-fail-mode', 'br-int', 'secure']