python-nova should depend on ebtables

Following an upgrade yesterday to python-nova 1:2013.1~rc1-0, we started getting "Interface eth1 not found" errors in nova-network.log (relevant logs below). This turned out to be a call to ebtables that was failing (because it wasn't installed).

There should either be a dependancy on ebtables or at least a more accurate error message (preferably both).

2013-03-27 07:18:24.812 28253 INFO nova.network.driver [-] Loading network driver 'nova.network.linux_net'
2013-03-27 07:18:24.827 28253 AUDIT nova.service [-] Starting network node (version 2013.1)
2013-03-27 07:18:27.043 28253 CRITICAL nova [-] Interface eth1 not found.
2013-03-27 07:18:27.043 28253 TRACE nova Traceback (most recent call last):
2013-03-27 07:18:27.043 28253 TRACE nova File "/usr/bin/nova-network", line 54, in <module>
2013-03-27 07:18:27.043 28253 TRACE nova service.wait()
2013-03-27 07:18:27.043 28253 TRACE nova File "/usr/lib/python2.7/dist-packages/nova/service.py", line 689, in wait
2013-03-27 07:18:27.043 28253 TRACE nova _launcher.wait()
2013-03-27 07:18:27.043 28253 TRACE nova File "/usr/lib/python2.7/dist-packages/nova/service.py", line 209, in wait
2013-03-27 07:18:27.043 28253 TRACE nova super(ServiceLauncher, self).wait()
2013-03-27 07:18:27.043 28253 TRACE nova File "/usr/lib/python2.7/dist-packages/nova/service.py", line 179, in wait
2013-03-27 07:18:27.043 28253 TRACE nova service.wait()
2013-03-27 07:18:27.043 28253 TRACE nova File "/usr/lib/python2.7/dist-packages/eventlet/greenthread.py", line 168, in wait
2013-03-27 07:18:27.043 28253 TRACE nova return self._exit_event.wait()
2013-03-27 07:18:27.043 28253 TRACE nova File "/usr/lib/python2.7/dist-packages/eventlet/event.py", line 116, in wait
2013-03-27 07:18:27.043 28253 TRACE nova return hubs.get_hub().switch()
2013-03-27 07:18:27.043 28253 TRACE nova File "/usr/lib/python2.7/dist-packages/eventlet/hubs/hub.py", line 187, in switch
2013-03-27 07:18:27.043 28253 TRACE nova return self.greenlet.switch()
2013-03-27 07:18:27.043 28253 TRACE nova File "/usr/lib/python2.7/dist-packages/eventlet/greenthread.py", line 194, in main
2013-03-27 07:18:27.043 28253 TRACE nova result = function(*args, **kwargs)
2013-03-27 07:18:27.043 28253 TRACE nova File "/usr/lib/python2.7/dist-packages/nova/service.py", line 147, in run_server
2013-03-27 07:18:27.043 28253 TRACE nova server.start()
2013-03-27 07:18:27.043 28253 TRACE nova File "/usr/lib/python2.7/dist-packages/nova/service.py", line 429, in start
2013-03-27 07:18:27.043 28253 TRACE nova self.manager.init_host()
2013-03-27 07:18:27.043 28253 TRACE nova File "/usr/lib/python2.7/dist-packages/nova/network/manager.py", line 1603, in init_host
2013-03-27 07:18:27.043 28253 TRACE nova self.init_host_floating_ips()
2013-03-27 07:18:27.043 28253 TRACE nova File "/usr/lib/python2.7/dist-packages/nova/network/floating_ips.py", line 98, in init_host_floating_ips
2013-03-27 07:18:27.043 28253 TRACE nova raise exception.NoFloatingIpInterface(interface=interface)
2013-03-27 07:18:27.043 28253 TRACE nova NoFloatingIpInterface: Interface eth1 not found.
2013-03-27 07:18:27.043 28253 TRACE nova