Have confluent log sessions out at exit

While the native perl threading object will do join() on exit,
eventlet's variant does not.  Fix this by manually hooking join() in
via atexit.  Since this is an eventlet specific thing, it makes sense
to work the issue in code that patches in eventlet rather than in pyghmi
itself.

confluent_server/confluent/plugins/hardwaremanagement/ipmi.py

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import atexit
 import confluent.exceptions as exc
 import confluent.interface.console as conapi
 import confluent.messages as msg
@@ -30,6 +31,12 @@ import socket
 console.session.select = eventlet.green.select
 console.session.threading = eventlet.green.threading
 
+
+def exithandler():
+    console.session.iothread.join()
+
+atexit.register(exithandler)
+
 _ipmiworkers = greenpool.GreenPool()
 
 _ipmithread = None