Fix keepalive behavior on broken Sessions

If a Session object is broken (e.g. BMC goes away or network outage),
then its keepalive timer would cause an application to be starved
of being able to do any IPMI related work.  Address this scenario
by having the instance deregister its keepalive at the point it
reports itself as broken.

Change-Id: I6b7af8ad03759c676110fb4f0b7bfa41642152f3

pyghmi/ipmi/private/session.py

@@ -1151,6 +1151,9 @@ class Session(object):
             self.ipmicallback(response)
             self.incommand = False
             self.nowait = False
+            # since our connection has failed retries
+            # deregister our keepalive facility
+            Session.keepalive_sessions.pop(self, None)
             return
         elif self.sessioncontext == 'FAILED':
             self.nowait = False