change logging message if no compute nodes available

[Christian Berendt]

please change the logging message to something more meaningful...

in /var/log/nova/scheduler.log:

2011-02-14 16:23:31,251 ERROR nova.root [-] Exception during message handling
(nova.root): TRACE: Traceback (most recent call last):
(nova.root): TRACE: File "/usr/lib64/python2.6/site-packages/nova/rpc.py", line 192, in receive
(nova.root): TRACE: rval = node_func(context=ctxt, **node_args)
(nova.root): TRACE: File "/usr/lib64/python2.6/site-packages/nova/scheduler/manager.py", line 63, in _schedule
(nova.root): TRACE: host = self.driver.schedule(elevated, topic, *args, **kwargs)
(nova.root): TRACE: File "/usr/lib64/python2.6/site-packages/nova/scheduler/chance.py", line 37, in schedule
(nova.root): TRACE: raise driver.NoValidHost(_("No hosts found"))
(nova.root): TRACE: NoValidHost: No hosts found
(nova.root): TRACE:

# euca-describe-instances
RESERVATION r-nrg6tjtv testing default
INSTANCE i-00000005 ami-3x776h8w scheduling None (testing, None) 0 m1.tiny 2011-02-14 16:23:31 unknown zone