Fix infinitely recursing custom keepalives

Custom keepalives are called regardless of whether a command is issued
or not.  The rationale being that custom keepalives are checking for
something specific rather than just assuring session state.  Notably,
SOL uses a custom keepalive to see if the payload is still active.
The resultant problem was that if keepalive expired just at the time
something was in the midst of a command, a session would infinitely
recurse into its own keepalive.  The issue was that the keepalive
expiry incorrectly omitted _monotonic_time, causing expiry to
always be far in the past.  It normally did not break because if
not incommand, send_payload was setting an appropriate value after
the incorrect setting.

Change-Id: Ie86e49890a6ac96ddf07206fb1b8558161c00a20

pyghmi/ipmi/private/session.py

@@ -922,8 +922,8 @@ class Session(object):
         sessionstokeepalive = []
         for session, parms in cls.keepalive_sessions.iteritems():
             if parms['timeout'] < curtime:
-                cls.keepalive_sessions[session]['timeout'] = 25 + \
-                    (random.random() * 4.9)
+                cls.keepalive_sessions[session]['timeout'] = \
+                    _monotonic_time() + 25 + (random.random() * 4.9)
                 sessionstokeepalive.append(session)
         for session in sessionstokeepalive:
             session._keepalive()