Tone down logging while waiting for conductor.

When nova-compute starts, it sits in a loop waiting for nova-conductor
to be responsive, since it is required for nova-compute to be able to
complete its startup process.  If a ping request times out, don't leave
a traceback mess in the log.  This is an expected type of failure that
is handled.  Just leave it as a warning in case the problem really is
that nova-conductor hasn't been started.

Fix bug 1137994.

Change-Id: I0754fbd6faf35a09b8733d3ced77871268abf1e8
Merge "Tone down logging while waiting for conductor."